Can I get a 4th axolotl? 40 gallon tank

[itiwhetu]

You are so going to need a 150g tank. These guys are going to grow and grow. Make sure you have space for these guys.

 

[OliveFish05]

You are so going to need a 150g tank. These guys are going to grow and grow. Make sure you have space for these guys.

I was under the impression that 3 full grown ones could live in a 36x18 inch tank? And I have plans to upgrade them to a 75 before they are all full grown. As soon as I know for sure which are males and which are females, they’ll be split up. I am just trying to figure out if this is alright while they are less than 8 inches?