Tortoises need much larger enclosures than an aquarium can provide unless you're building it yourself.

A 125 gallon tank is, in my opinion a poor use of dimensions for a bearded dragons I'd like to see 24 inches wide minimum, same for the skink but the skink would be less effected given it's more slender nature.

The diets are entirely incompatible as well. You can't have a bearded dragon or tortoise eating the things that a skink eats, they would get pretty sick over time.

The biggest problem is this, if for whatever reason a skink gets ahold of either of those animals it's going to kill the crap out of them. One of the most basic theories of community fish keeping is that everything needs to be equally nice or if that fails, equally matched. In this case you are putting an Oscar Chiclid Fish in with an Angel Fish, not in terms of overall aggression but in terms of fight power.

When skinks breed they can rip limbs off on accident. That's their love, imagine what's gonna happen if it gets mad at something.

Tortoises randomly bite as well which can easily cause broken or missing limbs in a beardie or skink.

Plus a 125 gallon is heavy as crap and difficult to work with and expensive as well. The cheapest I could find was 700 dollars.

You can get a 72x24x18 cage with the necessary light modifications from APcages for around 500 bucks for a bearded dragon and 48Lx24Dx12H for the skink is around 200 bucks after modifications.

My point is, the idea is a bad one. I know that some people have done similar things but quite frankly it's just not worth it and in my opinion those people are rare exceptions and the idea is stupid, at least with animals like the ones you're describing.

To me it sounds like you already have a 125 gallon tank and don't want to feel like you are "wasting" the room you are giving the skink because blue tongues aren't often recommended to have that large of an enclosure. If that's the case, I say just make your skinks 125 a really cool enclosure that everyone will be impressed with. Just have the skink in it. There is nothing wrong with keeping a reptile in a larger enclosure than it's supposed to have.