There is quite an age gap between the elder ones and the younger ones plus the big room is big enough for 3. I just want it to be fair for the eldest 2 as they're not siblings so don't want it to seem as if one is more important than the other iyswim
I'd ask them both privately which they would prefer. Might be one is quite happy with smaller room - often teenagers don't need a lot of space. My DSD moved voluntarily to smaller room when we moved in - she wanted a chance to redecorate and rid herself of her childish things (stickers on the wall, etc). It they're both intent on the larger one, then either give it to the older one (and the younger one gets his turn when older leaves home) or toss a coin.
3 in a room is a lot though - would you not be better with DS2 and 3 sharing?
How to fairly work out bedrooms. we have 5 ds' in a 4 bed house, 3 younger ones share the biggest room. The 2 teenagers will have their own rooms, smaller rooms. However one room is bigger than the other. Should we let them decide between themselves and if they want the same room toss a coin?