Studio One 3 was designed for 64-bit Mac OS and the minimum OS is 10.8.5, which is the first Mac OS that is all 64-bit. I have used Studio One 1 (32-bit) in 10.13 High Sierra and in 10.7 Lion. The performance in Lion was excellent but suffered severely in High Sierra, but it did work. You could try downloading some of the free demo songs that have a decent amount of tracks and effects and see how they perform.
The cost of upgrading to the newest Studio One 5 varies based on what version you have and what version you upgrade to. Artist 3 to Artist 5 would cost you $50 while upgrading to Pro 5 from Artist is $300.
You will be able to open the old version 3 song/project files in Studio One 5, but not the other way around.