Install Legscy T-RacKs and Sampletank?

[puranon]

AT4 contains all AMPS from AT3 and AT2, but is there any point to installing T-RackS 3 and Sampletank 2XT?

I believe there were some T-Racks3 plugins that are no longer available in T-Racks 5 and the programs like Samplemoog and Sampletron were made for Sampletank 2.x. and probably not even compatible with Sampletant 4 CS.

Also are the algos in T-Racks 3 and T-RackS 5 standalone plugin versions the same?

[DarkStar]

For the legacy "powered by ST2" libraries you can use them in ST4. You just need to go via ST3 (even ST3 CS). Please see
https://cgi.ikmultimedia.com/ikforum/viewtopic.php?f=12&t=24083

My understanding is that all the TR3 gear models are available in TR5. Which specific ones do you think are missing?

Install TR5 (even TR5 CS) authorise it. Then in the Custom Shop do a "Gear >> Restore my Gear". Then you will have full use of TR5 editions of all of your licensed TR3 gear. The TR5 editions are probably newer / improved / bug-fixed.