Hi there
I'm not surprised you've gone with the flow for your third LO, it's hard to fit in sleep training with other LOs to care for too. She's still tiny though!
Any routine you aim towards right now will take a little time to get settled on and to sleep train LO, I think you know that, so a routine is only a suggestion to aim towards. You would need to wing-it some and reduce A times and fit more naps in if she continues to short nap whilst you sleep train.
It think the first thing I'd look at would be the feed-to-sleep prop because it is likely to be hard to set out on the school run if you have to be feeding baby to get her off to sleep. Needing to F2S is probably already impacting on your ability to see DS1 off to school easily I would imagine.
Personally I'd probably opt to sleep train in the stroller so that you are able to get her out and back for the school run and mum and toddler groups etc. Sleep training into the stroller doesn't necessarily mean setting yourself up with another prop of long walks in the pouring rain to get her off to sleep, you can treat it as a stationary bed and use shush/pat or a little rocking and wean that with the aim of stopping before she nods off. That way she would learn to have a little shush/pat or rocking for her wind down and then be able to sleep in a motionless stroller.
Otherwise, quite a normal 3hr or 3.5hr EASY could be slotted around the school runs. Here is an example where I'm starting with a 1.5hr A time but I think you'll see this can increase to 1hr 45 by it just taking her a little longer to fall to sleep on the way to school or back. By the time she can manage a 2hr A time you'll probably be back home for that nap unless you take your 2yo to morning mum and toddler groups etc or shopping. It could be a good time to get a long stroller nap in.
Another thing to bare in mind is that if LO can't yet do a longer night you might end up with earlier waking in the morning if you shift BT earlier. It is hard to tell without a more regular routine with decent length naps.
So this is just an example of something to aim for
WU 7.00
E 7.00
A 1hr 30
S 8.30 - 10.00 sleeps in stroller/trailer/car - transfer into house on arriving home if possible (strollers are often able to lay flat so baby can sleep a full nap, car seats are not advised for full naps as the position is designed for protection in a car and not for best body position for development)
E 10 (or 10.30 on a 3.5hr EASY)
A 1hr 30
S 11.30 - 1.00
A 1hr30
E 1.00 (or 2.00 on a 3.5hr EASY)
S 2.30 - 4 put to sleep in stroller ready for school run (this S time will get a little later as A times increase)
E 4.00 (or just before CN on 3.5hr EASY)
A 1hr 30
S 5.30 - 6
E 7.00
BT 7.30
I hope something there is helpful for you.
