I would strongly, strongly advise against doing so. Big NOPE. Definitely not.
I know, on the outset, that it seems well and good to put your cow with your two donkeys--for all ends and purposes these could also be horses--because, to you, that lonely cow wouldn't be so lonely any more and have a couple of friends to hang around with.
There's just one small problem that is a much bigger deal than you may think: Bovines and equines don't get along all that well, and bovines are the ones that tend to suffer the most from such a relationship. This is especially true when equines outnumber, or are equal to, the number of bovines within the same corral. It's a bit of a different story when equines are grossly outnumbered by the bovine herd.
However, the situation with these two donkeys against that one cow is not good. I have seen for myself times when horses (donkeys will do this too) will gang up on one or a few cattle and chase them around for virtually no good reason except maybe for fun. This is what will happen to that cow. Those donkeys will find any little reason to harass that cow, nip at her, kick her, and generally make her life a living hell.
What's even worse is that these two donkeys will hog the feeding area all to themselves, and work together to keep that cow away, at all costs, from getting any hay or feed that they think is theirs and theirs alone. Equines don't share their stuff to any species they consider "lesser" to themselves. Any non-equine animal that tries to "steal" their food gets kicked or bitten, to the point where they learn that it's a lot safer to stay as far away from those nasty equines as possible until they see an opportunity to sneak in and snatch a bite--or two, if they're lucky.
The Bovine Practicum Q&A "blog" is an informative, just-for-fun section where I find a variety of questions that are often (and not-so-often) asked by inquisitive people like you, and answer them to the best of my ability. Much of the questions have come from a wide variety of reaches from the Internet.
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