So I have two sullys around 7/months that are kept separate. When they get larger I will obviously put them in the yard..,would it be ok to keep them together if I buy a third? Or best just keep them separate permanently?
Any new one will need to be quarantined for months, meaning a THIRD enclosure. To not do so is a pretty big gamble.
This will depend on their sexes. Three females will probably all get along fine. Two or three males and you are likely to see combat, in addition to the low level chronic stress.
I would not put them together for around a year and a half and only as 3. But since I wouldn't be able to tell the sex for a while....your right not worth the gamble.
If you are going to get 3 then I advise you split your yard into 3, only if you have enough space. But if you have all females then maybe it will be ok to house them together, although I wouldn't risk it
You can try it, however you'll need to do it on a day you have plenty of time to spend watching them. The best way is to introduce them in neither's yard. So you'll need a third yard, no matter if you get a third tortoise or not. That way neither tortoise will be trying to get the other out of "his" territory, as it will be a neutral territory.