We have 3 acres, 1 1 acre field of woodland, 2 fields of 1 acre of grass but only keep the ponies on one of them as the other and the wood are too wet in winter, has just dried out enough, so as of tomorrow we are harrowing and fertilising and re-seeding half of the top paddock the ponies will get the wood to browse for a couple of weeks instead, We almost never are able to graze the 3rd acre as it is very soft and wet, but we do manage to cut hay off it and briefly graze for a month or two. We suppliment all year with hay (ours and we cut 2 acres of a neighbours), and our 13.2 and 14 h live out 24/7. So yes I think you'll be fine on 1.9 acres with 3 - I'd section into two and rotate, and with a bit of hay supplimentation and good weather you'll be fine.
1.9 acres isn't that small is it? Am trying to imagine 5 acre field we used to have divided up to get an idea. Not for what is effectively two ponies anyway, since small one has to be kept off grass. Personally it wouldn't bother me too much about grass as I prefer to ration grass in summer and give a little hay anyway, much better for barefoot and lami types and very good doer cobs. It evens out costwise in the late autumn once the goodness has gone out of the saved grass because we don't have to feed much hay up until about Christmas. Less mud as well if it hasn't all been grazed down. We don't have a large field but we strip graze about half during the summer and save the rest. Seems to work ok. They don't need a lot of space in summer anyway as they work more and doss about sunbathing but they do like the run of the field for a bit of a spin when the weather is cold . If it was me, I'd get the extra pony, try to save some of the grass for later and resign myself to buying some hay during the summer (they don't really need much, not like in winter) plus spending a lot of time poo-picking! That way you have more options, for example if we have a nice autumn and small pony ends up staying longer, you won't go into winter with a completely ruined field. However, have just realised we are on chalk so very well-drained and we can get away with smallish paddocks if summer gets a bit damp. If your field tends to hold water I don't suppose any of this will apply .
If theyre all good doers then you should be fine, they aren't big so don't need quite as much space as 3 big horses would! However if one of them looses condition then extra forage/feed would be needed- something like fast fibre in the feeds of the ones than needed would be good as then the lami one doesnt have access to extra hay!
Ditto above about mud and winter- anything extra on the land is going to make it worse than usual, maybe consider strip grazing over the summer in order to manage lami pony and give patches of paddock a rest at different times so hopefully by winter you'll have a little bit of grass left!
And also make sure you do keep on top of poo picking and worming which i'm sure you will as you don't want any nosy passers by thinking you don't look after them just because you werent able to poo pick at the weekend- messy paddocks cause a lot of unecessary rspca calls that you just dont need!!
I would have thought you'd be ok grass wise, as long as we have what passes for normal weather. Too dry and you may be a bit short, too wet and it may get poached and mucky. Make sure you have access to an alternative, or have some hay just in case.
If it is hard grazed would it have time to recover for the winter? If it's very short going into winter you may find it turns into a mud pie very quickly...... Even a couple of weeks rest would help it.
We currently have dd1s 14.2 and dd2/dd3s 11.2. Little one lives in the field shelter as he gets lami. Dd2 has suddenly grown and now looks silly on him, plus she's found her confidence jumping and now can't be stopped!! he's 22 and on loan. Dd3 however is perfect on him. I was wondering if it would be possible to have another pony on my small paddock for the summer so dd3 can have the benefit of old pony. He won't winter out again so will go back to owners in September when maybe dd3 will be confident enough to ride the 13hh. I don't think 3 ponies on that acreage would work in the winter but for the summer... Of course then there's the transportation issue.. Any thoughts?