It will depend on where you are and what sort of land it is.
Generally, average to good land the rule of thumb is one acre per cow, two acres if you want to make your own hay. Dexters are small and thrifty so you might get away with two on 2.7 acres. But if it’s poor land - marshy, scrubby, not good pasture - then it might not support two. (And you can’t keep herd animals on their own.)
If the land is poor, if your winters are wet, then you will probably find that the land becomes a quagmire in winter. Cattle must have somewhere dry to lie, and need solid ground where you feed them, so this could be a problem.
Would you be able to erect a shelter for them? You might need planning permission.
Is the land classed as agricultural? Suitable for livestock? You might need planning permission (or equivalent) to put livestock on it. If it’s surrounded by housing you might not get it.
When your heifers have calves, how long would you keep them? Up to six months or so, with spring calving, you might get away with it. But two adults and two adolescents over winter, on 2.7ac... probably not.
All livestock needs to be checked, in daylight, every day. And early enough in the day that if action is required - eg, veterinary attention - you have daylight and working hours left in which to effect it. So in practise, they need checked in the morning, after sun up, every day. If you work 12 hours a day, can you do this? And if there is a problem, will you be able to deal with it? Would you be able to call work and say you have to stay back to sort out a cow issue?
Hay-wise, depending on where you are and the winters you get, you might need to feed hay for four months or more. A full-size suckler cow with good shelter would get half a bale of hay a day, so your two Dexters might need 100 bales, maybe more.
There’s no reason a horse couldn’t share with cattle - but a) all herd animals should have animals of their own kind for company, and b) 2.7ac, even if very good ground, wouldn’t support two Dexters and a horse other than in the very best growing weather. Last year, we had an evil winter, a cold, slow spring, then two months’ drought. We had no grass until September / October. Farms around us were feeding hay in August. I’m in North Cornwall.