Hello. Congratulations on your new arrival and welcome back to BW

With regards to A times for age and from my experience on the boards I have seen prem babies work better with an A time closer to their adjusted age for many months but gradually moving more towards their actual age. The shift towards actual age could be when LO is closer to 10-12 months old but that's just based on what I've seen here and as you know every baby is different and we always look at individual needs rather than a one size fits all schedule.
I would say then that your LO looks like she's on an A time for roughly 3 - 4 months which fits her adjusted age - and has a shorter first A time which is fine, many do like that.
If you feel the A time is too short and causing the short naps then go ahead and try another 10 or 15 mins A time and see how it goes for a few days. Perhaps just extend one A time (maybe the second A time if she likes that first to be short) and see if she settles into it. You may well see an OT nap there (20-30 mins rather than 45) and need to resettle her but these could stop after a few days and the nap become longer and more restorative. I would keep a record whilst you try it out so you can look back over what's happened.
Also for short naps, have your tried using W2S for a few days?
https://babywhispererforums.com/index.php?topic=223809.0Look at naps option 1.
This can help LOs learn to link those sleep cycles.
You could for instance try it just on 1 nap per day (the same nap each day to give consistency and build habit) and see if it helps.
And should she still be on a 3 hour routine or should I look at moving to a 4 hourly?
This will be totally reliant on how often she neds to feed. Tracy said some babies do not move to 4hrly milk feeds until 6 months, the examples included prem and low birth weight babies, some BF babies, but mine was none of those and he still couldn't move past 3hrly E until 6 months old when he had solids come between so it wasn't a long wait for food.
I would not try to extend her E times if she is taking a good full feed every 3hrs. If she seems less hungry at 3hrs and either refuses to eat for a while or doesn't take a full feed then that would indicate readiness to increase the time between E. Also you do not need to move right up to 4hrly E, you could shift to 3hr 15, 3hr 30 or whatever suits.
If she naps beyond her E time this will naturally extend the E and that's fine.
I hope this helps. Let me know if you have more questions or how you get on.