How many to a colony...
Depends on the size of cage, and the gliders.
The following is not meant to be judgmental and isn't JUST for the Op, but also anyone else who may be reading this and has a similar situation.
Are the 3 girls still being housed in the pp starter cage?
If so, I'd suggest instead of another glider you invest in a bigger cage for your current gliders.
The pp starter cage is only meant to be used for about 6 months for 1 or 2 max gliders during the initial bonding period. Then it's suggested to upgrade to a larger long term cage.
A larger cage may also help you with bonding with the girls. How?
with more room the gliders will be less stressed and they will be able to move more and spend more of their energy at night which can make them calmer and easier to handle during the day. You'd also be able to fit a glider safe wheel (and other stimulating toys) in a larger cage providing an even more positive outlet for exercising and spending energy.
If they are still in the pp cage and you haven't already done this, you can remove the shelf and ramp in the cage to free up some space until they get a larger cage. The ramp and shelf aren't necessary and take up 'a lot of valuable space. It is still not a long term cage for 3 gliders but will help temporarily.
For 1-4(max) gliders I like the brisbane cage, though others would say for 3 or more to use a double critter nation (DCN) for more gliders add more space, Dcn's can be made into tripple stacks or
you can take two dcn's and put them together to make an even bigger cage for larger colonies.
How many to a colony.....depends on the gliders.
Even if the cage size is adequate for a large colony it ultimately comes down to the gliders, much like everything else.
Some gliders do better in pairs or trios rather than larger colonies, despite being colony animals. And some gliders do great in large colonies (4 or more) Even if the introduction goes extremely well the colony can collapse once housed in the same cage. So you have to monitor how they do and their behavior for quit a while after intro's.
They may thrive with another glider and get along great, or the new glider can cause i rift with the established working trio.
It's a 50 50 chance either way.
Reputable breeders in Texas.
There are a few reputable breeders in Tx, but the chances of them selling to someone that lives in an illegal state is very low unless you lie, and they believe you. It's not just that you live in an illegal state though for many breeders that is reason enough in itself. It's also that you will have a serious disadvantage for good vet care in your illegal state.
Good vet care is hard enough to get in legal states, let alone illegal states with even more inexperienced vets.
Vets in illegal states can and HAVE turned in people with illegal pets, the pets pay the ultimate price for the owners decision to have them in illegal states, most often the pets are put down whether they are healthy or not.
Which makes owners want to avoid taking gliders in for medical attention when they need it, there again the pet is suffering for the owners decision. Some vets may overlook the fact that they are illegal and treat the glider as best they can, but most ethical vets feel they have an obligation to turn owners in.
No reputable breederis going to subject their gliders to that, if they do knowingly then they aren't considered reputable.
If they aren't reputable, chances are they aren't breeding ethically or responsibly either.
So then your looking at possible inbreeding and or possibly selling sick gliders. Who then are going to need.......vet care at some point. Not to mention you'd be subjecting your current healthy gliders to a potentially unhealthy glider.
None of this is to judge you on your choice to own gliders in an illegal state, however these things are just some of the reality and consequences and limitations of that choice. You may have considered all this, but someone else
reading may not have.
No one can stop you from getting another glider, but first I'd focus on the ones you already have.
There is never a stopping point for "bonding" the bond continues to grow and expand as long as you own them no different than a friendship grows over
time with 2 humans. I really do believe if you focus on them, get them a larger cage and stimulating accessories you will see a difference and your bonding is going to progress with them easier.