its a houseplant.. there is such a forum.. if not the name that plant forum.. should we not be able to ID it here ... after that.. take the latin name.. add PROPAGATION to such.. and google the answer .. most of this stuff.. if you cut those trunks to 3 to 6 inches.. of varying height.. and just properly water it.. it will rebud into a nicer shape ... and many of us.. would repot it at the same time.. potting media wears out.. as to whether you want/need... to double the number you have.. well.. that would be up to you ... ken

Oh dear, I wish you had asked here first ;-) Never mind, it's done now and all you can do is wait for it to regrow. Firstly, 12 feet is not particularly large for a lilac -they can make nice little multi-stemmed trees and a mature specimen will indeed have thick branches. But that is not by definition a bad thing if it is healthy. If you want smaller lilacs it might be useful to research some smaller cultivars which will stay naturally nearer the size you want. Secondly, cutting off at 3 feet will give you a shrub on legs rather than growing full to the ground. The best way of dealing with it would have either been to cut it all to the ground and sacrifice all flowers for a few years or to cut out a proportion of stems to the ground over two or three years thus renewing all the stems eventually. You'll have to decide if you want your lilac to remain up on legs or to take it down to the ground. Either way they will sprout again so don't worry about that.

Right^. The use of the word 'severely' by definition indicates an overzealous attack on the trees. They will-as trees always have-respond to the loss of numerous branches by attempting to right this loss. We call these suckers. Where there was one branch before, now there might be sixteen, all trying to develop around the same growing point. Much does depend on species, but when you say severely, no, that won't work for anything. Might coppicing be an option? That way, you'd end up working with all new stems, not mangled and abused things. For trees which are still basically in good shape but which have been long neglected, the "crown cleaning" that may be needed should be done taking into consideration that the tree has not had work done to it in a long time, and that therefore, they should be brought around gradually. Anything which lops off lots of foliage at any one time is likely to do more harm than good, but many tree species will resprout from the complete loss of above-ground parts.

From the looks of it I would say the red colored growth is very likely the root stock. The Budagovsky (B9), (B118) all have that typical coloration. Looks like the far left and possibly the far right trunks are root stock from the picture. I am sure you can tell from the leaves. I would go ahead and remove those 2 trunks now. IMO I would leave the "regular" apple now and prune out a trunk or two next dormant pruning. I would not rip out the tree and start over looks to be healthy and it can be pruned into a more productive tree if you like the taste of the apples. Otherwise you can graft it over to something else.