234 definately works for when you move to 2-nap days. The more pertinent question would be is baby ready for 2-nap days. Once she us then 234 is the way to go (Or some slight variation within - for example 224 or 2.5/2.5/4)
Whether baby is ready for 2-nap days comes down to nap length. As nap length increases then awake time between naps can increase.
A 1h nap would probably allow for 2h awake time. You'd need a 90m nap (at least) for baby to manage 3h awake.
To manage that longer 4h awake spell before bedtime, you'd want baby having had around 4h sleep total that day. This is arrived at through basic maths - 2+3+4 means 9h awake during the day. If sleeping through the night then you should be getting around 11h sleep at night (in the 10-12h range). So total daytime sleep wants to be in the region of 4h total - less if less night time sleep, more if more night time sleep.
So say baby wakes after a good night's sleep at 7am. I'd expect to keep baby awake until 9am. At this age I would start having first nap at a routined time.
The rest of the day is judged on nap lengths.
If baby naps for 1h (9-10am) then it follows that awake time will be 2h so next nap at 12pm. If that's 1h (12-1) then we would need a 3rd nap at 3pm to get through to bedtime.
If baby naps for 1.5h (9-10.30am) then you could go for 3h awake time so next nap at 1pm. You've then got to see how long that 2nd nap is to judge if you can try for a 2-nap day.
- if baby sleeps 90m (1-2.30pm) then chances are you'll need a teatime power nap.
- if baby has a long nap, say 1-3.30 then defo go through to a 7.30pm bedtime.
- if baby does 2h then see how things go. It might be that you do an early bedtime, for example 1-3pm nap but baby is ready for bed by 6-6.30pm. But this may then have a knock-on effect the following day with wake up time. So it might be better to do a 2.5h awake time, allow for a 5.30pm power nap and a later bedtime.
- if second nap is more like 2h then as above, you might need to either slot in a teatime power nap or do an earlier bedtime.
This transition period from 3-nap days to 2-nap days is always a drawn out thing. They'll be days of long naps, days of shorter naps, days of early bedtime, days of 3 naps and later bedtime. Just follow your baby's lead and work on awake times. As a general rule awake time wants to be around about double the previous nap length (give or take). It will eventually draw out on its own.