You get to decide how to train or allow these plants to either become a small patio tree, an under story tree or if you want a bush.
Thin only for a tree. Head for a bush.
For a tree make sure you never cut the tips of the branches off. If you want a bush, you use hedging shears and cut off all the tips, the apical, terminal buds keeping your shears tangent to the 'sphere' of the shrub. The form should always be wider at the bottom and narrower at the top. Keep you shears tangent to the 'salad bowl' you are using as a template. The wider and flatter the bowl the better. Cut those off and all of the energy you cut off is transferred down the stem into the lateral buds. Creating bushy.
All Crepe Myrtles are either large shrubs or small trees. Would need a microscope and a frozen flower to ID the species. Dunno if they've made a 'shrub' Crepe Myrtle on purpose.
Crepe Myrtle has one of the loveliest forms of small trees, Vase shaped, sometimes multi trunked, perfectly sized patio tree with an explosion of color that takes your breath away. I know you Texans take it for granted, grins! You are so lucky to have Crepe Myrtles every where. I love the small tree form that forms an umbrella over the patio. Tough to find multi trunked specimens I've found. Perfect to make a powerful 'out door' room.
If used as a shrub you need groups of them to add anything to your landscape. Just one here and there used as a shrub would not be as important as a larger, tree specimen.
Think of Bonsai. Small patio trees should look like humongous Bonsai creating a 'ceiling' as well as 'walls' and they actually MAKE a patio, a room that is human scale. Add a bit of pea gravel a comfortable chair a candle and that room formed beneath the canopy becomes a memory.
The first two look the same and I think the third one isn't getting the light, water or something. Look into how to prune trees, THINNING versus heading. You have lots and lots of time.
Additional notes:
Well, I guessed wrong on Crepe Myrtles...I love them even more! Ground covers? After 2 1/2 years the two that are already large shrubs/small trees should become a tree. Is the first picture the runt of the three? What shrubs are behind this plant and are planted along your foundation? I see a spot that screams for a shrub, grins. To hide that utility box? Those two shrubs are planted too close to the foundation but not a big deal. If and when you do plant another shrub on that corner, plant it a bit farther from the foundation and to the front and left of that box. Make an isosceles triangle between the three shrubs if possible.
After seeing the VAST array of sizes, I doubt I can help you identify your plants! What I would do, if these were my plants is go to large nurseries with pictures and a sample of flower/leaves/buds. Perhaps even wholesale nurseries not just the retail. You need someone very familiar with Crepe Myrtles.
Have you been introduced to your local Cooperative Extension Service of one of your Universities? They choose which University to Host a Coop. Lots of satellite offices as well all around your state.
If a Botany student taking Plant ID at a University had a flower, a bud, leaves of your three plants using microscopes to see the necessary parts of the flowers, they could ID Genus, Species and some varieties.
For Free?
I think it is time you get to know your Cooperative Extension Service! They will know Texas' Crepe Myrtles better than anyone else. For a gardener that knows as much as you do, their Master Gardener Program is a MUST. Almost free, only takes a few months, most is done on the internet now as well. Lots of gardener friends and a massive network!
