i usually keep 2-3 females to 1 male with jacksons, then separate the females as soon as they become gravid..always worked for me..although my smallest enclosure i ever used for "sets" was 7 foot tall by four foot by four foot ..has always worked for me..
Everyone has told you 1 cham in an enclosure that size. Yet you still are asking the same question hoping for a diff answer. We can say yes keep 3 jacksons in there all day long to appease you. It still wouldnt be the RIGHT answer. Stop and think whats best for the animals not what you'd like. If you want your chams to survive and thrive in captivity there are going to be certain things you will have to be willing to do to keep them healthy. One of the most imp things is keeping the animals stress to a minimum. A stressed cham wont eat or bask normally. Thats a great way to slowly kill a healthy cham. With more than one cham in an enclosure that size your going to see stress in both or all three animals. The strongest of the group will survive the others wont. Or possibly all three could perish. Not worth the risk IMO.
If the Op is diligent and watches temps and drainage issues they could make the exo terra work. They are in Canada, so temps should be cooler than ours down here. But the whole 3 chams in an exo terra is asking for problems.