If you try and split 2 beasts one into a trailer and one left in the field both are going to get upset
I'd assumed the new one would be bought before the oldest was taken away. So there'd never be one in the field. Because there'd never be one in the field; any beast left alone would break out, or damage itself trying. But this isn't ideal either, the bigger older two would be bullying the youngster until the oldest was taken away.
I haven't quite got my mind around the age logistics either. Assuming you'd want the oldest away before it was 30 months, you'd need to be buying your youngster at under 6 months old. Suckler calves are usually kept on their dams until 8 months, aren't they? So you'd be getting it weaned early, just to suit the one-in one-out model. Not sure I think that makes sense, to either you or the breeder. And a 6 mo old calf joining a 'herd' of an 18 mo teenager and a 30 mo great big adult...
Not very sure about it, at all.
You could look at buying in younger beef x calves from a dairy herd, and bucket-rearing, but you'd need to get two or more at a time. Then you could sell one and butch one, or something like that? But you'd need to be good at picking the hardy ones, many half dairy types wouldn't outwinter, certainly not at under 12 mo.
Could you winter them in the shed, let them out when the weather's moderate?