I'd probably lean towards having your older two in together. And like Tegam, i don'tthink having different sexes sharing is a problem - they'll let you know when it's no longer ok.

Having said that, the best laid plans, yadda yadda... we had two boys in together when I fell pregnant with Pie, we spearated them & set up the bedroom for our youngest to share with the baby (he was 7).

She's 2 and a half and still in our room