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.

google: rejuvenation pruning of flowering shrubs ...

no need to add lilac to the search term...

ken

Ditto to comment one. Id finish cutting them back. They'll be less of an eyesore. However are they just too big for the spot?. If they are you'll spend your life cutting back and waiting for them to re-grow.

And learn from us old time, experienced mistake-makers: before you pick up the pruning shears pick up a book or some advice from the internet or Gardenweb. Look twice, cut once