Asking professor about certainty of a new opportunity
I might consider a few other options rather than waiting for something. 

That sounds like a good idea to me in any case! Probably nobody can give you a quantitative information concerning the probability to get the position (how would anyone know the probability that two other candidates decline an offer?). Moreover this information may not be very useful or verifiable (what do you do if they tell you "it's about 30%"?). If a tangible offer is presented to you; like a contract that is ready to sign, then you may have a reason to be congratulated. Until then I wouldn't give any importance to that email.

I suspect that this group wants to be sure to obtain a student within a certain time (else the funding may expire), but you're apparently not their first choice; so they might be trying to put you 'on hold' as they may fear that their preferred candidates take other offers, and that in the meantime you could have found another good opportunity, too, leaving them empty-handed. That does not sound very fair to me; even though it is understandable from their perspective. I think that the email should have contained information on a deadline, stating that the final decision will be made before a specific date. 

Just my two cents: since I don't think that you will obtain any useful answer, I would probably not ask anything and look for other opportunities instead. Maybe in a few weeks they make you an offer; then you can still decide. But I wouldn't recommend to wait and hope.