There is no hard and fast rule so often it depends on the space and the style of the pendant so post at least a pic of the pendant you would like to use . I for sure like a pendant to be LED 4000K to properly light that large island and usually odd number of pendants is a nicer look. You have quite a large island so in my head 18" is okay but it would be nice to see the kitchen and the lights you are considering.

Here are some guidelines.

ceiling is 12'

Light is 18" in diameter and 11.5" tall. It also comes in 17"w X 17"h or 12"w X 16.5" h

Using the guideline I posted: 126”-16” (minimum 8” clearance on each side) - 54” (pendant width) = 56” divided by two gives you 28” space between the pendants (1.5x width). I would say it’s a little tight. Ceiling height will be a factor, because they will look extra squatty if you have an 8’ ceiling with big domes.

Ceiling height is 12'

This is the room and island size with a 12' ceiling

Island is 54" wide and 126" long.

Think twice about an island almost 5' across. It'll be very difficult to keep it clean in the middle.

I would like to have three pendants but I'm not sure how large they should be. I preferr dome pendants.

You've been given lots of "rules of thumb", and they're a great starting place ... but you should "try before you buy". Buy a pack of big balloons and blow them up to the approximate size you're considering. Start with the suggested sizes and hang three, hang four, hang big, hang littles ... until you find a configuration that pleases you.

18" is very large.

I preferr dome pendants.

I like things to be easy to care for, so I suggest you go with pendants with an "open bottom" so bugs won't collect in the bottoms.

ceiling is 12'
Think hard about window sizes, window coverings, empty spaces near the ceiling, changing light bulbs, cleaning cobwebs. Have you seen this in person?
You cannot have everything be a star. Pick the hood or the lighting to be the supporting player.

Good advice.