Jacaranda-semi evergreen, BEAUTIFUL blue flowers, open canopy. A bit messy at leaf drop.Feathery leaves. It can be multi trunking or a single. No fragrance. Drought tolerant.

If you can tolerate a deciduous, Crepe Myrtles are one of the most gorgeous multi-trunked tree I've seen. The ones with Indian names have great resistance to mildew for those in damp zones.
Most 'multis' I can think of are deciduous - Tulip Magnolia, Saucer Magnolia....
Arbutus unedo is evergreen but may not fit your other landscaping. Ditto for Olive.
I have seen Oleanders trained as multi-trunks and very attractive.
Chitalpa is one to google - heard excellent things of this one.

Western Redbud, but no fragrance.

The Arbutus Marina is the most beautiful tree all year around. If it has a fault it may drop small fruits occasionally. It has a bloom that attracts hummingbirds but is not showy. If pruned to multitrunk the slick red structure is stunning all year as is the deep green foliage. It does not require summer water or spraying, ever.
The Chitalpa is also beautiful with a very long bloom season of showy flowers. It is however deciduous and drops its leaves over a long season. Al

Not typically grown as a multi-trunk tree, but I'd suggest Sweetshade tree, Hymenosporum flavum has all the other desired characteristics. It is fragrant, (they don't call it the Australian Frangipani for nothing), evergreen, doesn't get too dense or wide, is open in foliage so that it really is a see-through type canopy, and could be planted as a group of two or three close together, perhaps 8 to 10 feet apart. It does not handle high winds well, so if this is an issue in your proposed location, I would suggest not using it.

Jacaranda trees are beautiful, but they are also messy and prone to easily breaking branches in windy spots, and can get too big for most gardens, and typically are not open habit until they get much older.

Marina Madrone is definitely a beautiful tree, I used to have one in front of my own house, but it is not the best candidate if the dropping messy fruit and flowers will easily be tracked into the house, or will extend over pavement which it will stain.

Another good smaller tree candidate that fits into most gardens without getting too large, might include Tristaniopsis laurina, the Swamp Myrtle. Showy patchy bark like a Sycamore, attractive evergreen foliage that can be thinned out if you want it more open, doesn't get too big, and drought and wind tolerant while also having somewhat showy golden yellow flowers, but not fragrant, if I recall correctly.

I'd also throw out another candidate for a beautiful and well behaved tree for the SF East Bay, the deciduous Chinese Pistache. Not evergreen, but beautiful fall color, well behaved roots and low water needs, and good scale for a typical residential garden setting.

If it has to be an evergreen, I'd also suggest that Acacia cognata or Agonis flexuosa 'After Dark' are interesting choices for a somewhat weeping form, interesting foliage, and moderate water needs and ultimate size. Neither are typically available as multi-trunk specimens, though.

Weeping bottlebrush: evergreen, you can plant three together so they look multitrunked, flowers for much of the year, weeping habit, not dense, easy to grow and drought tolerant, I don't know if they smell good though.

Cane's Hybrid Callistemon is pink!
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Renee

Renee,
A beautiful photo of that pink flowering Callistemon, but I don't know how you could possibly describe any of the tree-like weeping Callistemon species and cultivars as not being densely branched... In my experience, they are amongst the most densely branched trees/shrubs that I can think of, and take a lot of time to thin out if one prefers a more open look. Perhaps C. viminalis is slightly less densely branched than C. citrinus species and cultivars, but still not what I would call a see-through foliage habit.

I think that a Pineapple Guava could fulfill all your requirements.

http://hort.ifas.ufl.edu/treesandpowerlines/feijoa_sellowiana_spring.shtml

http://www.motherearthlandscaping.com/plant_search/plant93.html

easy, evergreen,

pretty flowers:

http://image03.webshots.com/3/5/90/66/12759066eJSsoEiMpv_ph.jpg

smells good, can be multi trunked, drops fruit in the winter, but well, yum, extra added bonus.

Nice silvery leaves are quite ornamental.

Purple smoke tree, I think they keep their leaves, but not sure. They are beautiful, very airy.

Here is a link that might be useful: {{gwi:567700}}

The smoke tree foliage is beautiful but deciduous. The Tristania laurina is evergreen and has attractive peeling brown bark which exposes a smooth white wood. In our climate it does not get too big and is often used as a street tree. Al

Bahia,
Wishful thinking?
Renee

Flowering and "not messy" are kind of incompatible. Redbud is horribly messy--the seedpods go everywhere and the seedlings are really difficult to dig out (taproots). Stay away from the pea family.

How about a tree aloe, Aloe'Hercules'. Clean, flowering, (hummers love it), multi-trunked, airy, easy to grow, evergreen...how cold are your winters? Probably only hardy to around 26F.

Or maybe native Manzanita Arctostaphylos glauca. Like an Arbutus but without the messy fruit. Hummingbird magnet. Little bell-like flowers. Evergreen. Beautiful multistemmed structure with deep red bark. Needs good drainage, but little water required.

May I suggest:

Garrya elliptica, coast silk tassel. Everything you want except scent.

Calycanthus occidentalis, western spice bush. More like a large shrub, but can be pruned into a multistemmed tree. Beautiful, fragrant flowers.

Both local natives, drought tolerant.

Victorian Box --- Pittosporum undulatum

Pittosporum undulatum is a good tree in the right place, but by no means could you say this tree isn't messy. I have 3 of them in my own garden, and they drop sticky seed pods, and are also pretty densely foliaged canopy trees. They do look nice, have attractive orange fruits, and the fragrance is wonderful, but I wouldn't recommend them per the requirements of the O.P. Plus, if you aren't in a warmer part of the SF Bay Area, they aren't reliably hardy in places that drop much below 28F on a regular basis in winter, and can be killed at 24F as they were here in Berkeley in the 1990/91 freeze.