· John Heider's Answer
· Hi Sandra-If you are sure your trees are the Dynamite or another larger variety that grows to 15 or 20 feet in height they can definitely be pruned into a nicely shaped multitrunk tree. If they happen to be a simi-dwarf variety that gets 8 to 10 feet in height they can still be shaped into a nice small tree. Some of the smaller bush forms need very little pruning if any and would not make a nice small tree as you desire. If you can upload a picture of your crape myrtles it may help to determine what correct pruning should be done. Above this answer and to the right of your name at the bottom of your question you will see where you can upload your picture.
If you would like a multi-trunk tree with 3 or four trunks you would want to select the 3 or 4 best, most upright growing ones. After selecting the best trunks remove all the rest, cutting them off as low as possible to the ground. This heavy pruning should be done in late winter before new growth appears in early spring. After removing the unwanted trunks you can remove lower branches along the remaining trunks to a point about halfway up the trunks. Take care that you don't remove any branches that might spoil the shape of the canopy you are trying to develop. If the branches have grown tall and spindly you can prune the top of these branches. I noted below a link to a good article in Gardenality written by Brent Wilson that will show you how to prune these upper branches. This article will show you also how to prune each year in order to develop the nice canopy shape you will want for your trees.
This information above is noted in several answered questions other members have asked regarding pruning the crape myrtle into a nice tree form. Brent and Brooks Wilson are very knowledgable regarding crape myrtles and have answered many questions as yours on correct pruning of crape myrtles into a tree form. By clicking on the 'Ask Expert' tab above and typing in Crape Myrtle in the search box you can find these questions and answers and others that may be interesting to you also.http://www.gardenality.com/Articles/344/How-To-Info/Pruning/How-To-Prune-A-Crape-Myrtle/How-To-Prune-A-Crape-Myrtle.html
Hopefully this has helped. Please ask if you have any other questions. If you upload a picture of your crape myrtles I will be notified and will get back to you.