Hi
Yes, I did short-long-short with my DS, but out of necessity rather than choice as he would only ever do a short first nap, and not until he was 5 or 6mo and managing 2.5-3hr A times. Like you, the only one I could easily resettle was the lunchtime one. From memory our day was roughly:
Wu: 6.30 ish
A- 2.5hrs
Nap 1- 9 ish for 30-40 mins (depending on how long he slept)
A 2.5hrs
Nap 2: 12/12.30 for 1.5-2hrs
A: 2.5hrs ish
Nap 3: 4-4.30 PM
A:2.5hr
BT: 7pm
I seem to remember that he gradually increased his first and second A times so naps 1 & 2 got later but for a while he would always be ready for a CN at 4, even if he'd only woken at 3! Gradually that got later and shorter too & also harder to get him to settle for it until I couldn't even AP it and he went on to a short am/long pm routine.
On other hand DD started the same- only easily resettling lunchtime nap, but as her A times increased she started doing a long morning & afternoon nap herself. This was also around 5-6 mo. Around 8mo I started capping the am nap as the pm one was getting later and later and harder for us to ever get out the house as DS' rest time was her main middle of the day awake time!
With your own DS2, I think Lindsay is right that it feels too early to start capping naps. Around the same ages as your LOs, I think I just ran with the naps we got, some short, some long. As she says, do you want to post your current EASY to chat it through?