150 uk gal standusing 3x2 is it wise?

The safety depends on how you build it. Personally I wouldn't trust 3x2 for a 24" span with 36" of water on top of it, so you might want to use something a bit thicker for the top frame that supports the base of the aquarium (don't under-estimate the need for rigidity here, even a tiny amount of deflection, <1mm, will transfer the load onto the glass rather than the supporting member). You could the the 3x2 for the legs, especially with Joeys design where they are effectively doubled up, but with a tank of that height you would probably want to use more legs than you would with 4x2.

Time Lord 1 said:

So just to confirm, im better off building the part the tank will sit on with the 4x4 and the thinner stuff for the legs but beef it up more?

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Absolutely. The load through the legs is pressing straight downwards, unless the legs are way too skinny (which I doubt the two 3x2 together would be) or are warped/bowed I can't see any problems. However, the horizontal members that support the base of the tank need to be far more rigid as they have to transfer the load from the centre of the span to the edge/legs without flexing. For example, the steel used for my 2000L tank stand is 50x50x3mm for the base and legs, 100x50x3 for the front and rear section (max unsupported span of 537.5mm) and the front-to-back members are 100x50x5mm for the 1100mm unsupported span. The longer the gap the stronger/more rigid the material needs to be.

I'm no expert on timber strength, but for a 3' high tank I would think 4x2 would be the absolute minimum for a 2' unsupported span, and personally I would make sure it is good structural grade timber or some sort of hardwood.

Not sure if it will be as strong as, but would probably be similar. I think you would want to sand/machine the faces smooth and use some sort of adhesive similar to what they use for plywood to truly laminate them together. Laminated beams aren't uncommon in construction, but there is a bit more to it than sticking a couple of bits of wood together!