So, my character has a donkey. I'd like my donkey to be 3 smart, so that I can use telepathic bond to talk to it.
There aren't stats for donkeys. There are stats for various horses, and for mules, and even for a wooden donkey; all have an intelligence of 2. That makes a great argument for saying that a donkey is 2 smart. Of course, a @#%# cat is 3 smart, and I am pretty sure my donkey is smarter than any stupid old cat. But how to convince the DM? Are there any in-game statistics or comparisons that I've overlooked that support assigning a donkey an intelligence of 3?
Failing that, is there there a way to give my 2 smart donkey a boost to 3? I have searched, but I'm not coming up with anything. Some sort of elixr of smartness? Temporary would be nice, so that I don't have to put up with a smart ass all the time.
You need an Ioun Stone of Intellect.
You've cited the relevant material for determining the canonical intelligence of a donkey. Horses have an intelligence of 2, and if you've ever met a donkey, you know that giving the donkey a 2 is generous. That said, we do have a method for applying a modest intelligence boost to the donkey.
It may be difficult to get your dumb ass to do this, but it should work. This answer establishes that it is possible for animals to attune to magic items. Fellow donkey handler and stack user Pyrotechnical suggested in a comment: "Put the ioun stone in a salt lick to keep the donkey focused on it for the necessary hour." If you can get the donkey to attune to the Ioun Stone, we have overcome the first obstacle.
The second obstacle has to do with the stone itself:
The donkey needs to toss the attuned stone into the air to begin benefiting from it. This can probably be achieved by balancing the stone on the donkey's snout and kicking it, causing the snout to rise rapidly, tossing the stone into the air.
If done correctly, these steps will confer your donkey with the following benefit:
Your donkey is now 2+2=4 smart.
When you need to end the effect, returning your donkey to it's humble 2 smart state, you can grab it:
Notably, this separation will not end the donkey's attunement to the stone, so to reapply the intelligence boost, you will only have to redo the snout toss trick.
Headband of Intellect: How smart is too smart?
The Headband of Intellect is another option, but this may be way overshooting your goal here:
Do you really want a donkey with an intelligence of 19 when you could give yourself an intelligence of 19? Just think, this could be you: