Aehr Test Systems (AEHR), Keysight Technologies (KEYS) and Teradyne (TER), which one is a better equipment test company to buy?

Would like to hear industry experts advise as I am from another totally different field and background.

To me, all 3 are making money off contracts to test semiconductor, but after loads of reading, I still can't spot the difference between the 3 or the subtle nuances