Heya! I agree with Jp!
The general rule to follow (to keep an axie comfortable, water parameters good etc), is 40L of water and at least a 30x30 cm squared area of floor space, so for three you'd be looking at 120L of water and a 3ft/90cm long by 30cm wide tank as a good place to live.
A better guide, however, is how much spare area you've got in your house. Buy a tank to fit that area and you're good to go, as tanks are addictive and you'll undoubtedly end up wanting a larger one or more- I ran into this problem, going from 20L (with juvies and then with the australian summer
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) to a 200L/ 120X 35X45cm tank, and now I'm in the process of upgrading to a 650L/ 180x60x60cm tank
Another thing to consider is do you think your family of axies is going to grow? I started with three (Freddie, George (Who passed away nearly a year ago) and Phree), and I now have 11 (Six of whom I raised from two weeks old and stayed with me when other people decided it wasn't the right time for them to add an axie into their lives- not that I'm complaining in the slightest!!
Also, heads up- Please please please don't use gravel as a substrate!! If it's allowed to free float in a tank, it's a guaranteed impaction in the making.