The only one who has a realistic shot about it is Pogacar. He would have already done it if it wasn't for Van Der Poel. But he is going to have a hard time to win both, especially San Remo, which will be extremely hard unless VDP misses one edition.
As for the others - I can't see Van der Poel winning Lombardia. LBL is a tough ask, but could be in his reach. Anyways he would need a decimated field and, of course, no Pogacar or Evenepoel.
Evenepoel can definitely win Lombardia, could also win RVV if he improves his technique in cobbles (looked better in Brabantse Pijl than in his previous attempt), as well as Roubaix and San Remo, but I see to many "ifs" and unlikely scenarios to see it happeing.
Van Aert is definitely impossible. He has the best allround ability behind Pogacar and he has won San Remo, which is arguably the hardest to obtain, but the guy is unable to land big wins even when has been the best of the field... Now he is clearly behind Pogacar, Van Der Poel and not superior than Pedersen, so very little chances
Something like this:
Pogacar - 60%
Van der Poel - 10%
Evenepoel - 5%
Van Aert - 0.01%