Options for re-purpose F5 3600's

3600 Appliance supports up to BigIP 12.1.2 software. Check HW/SW compatibility matrix for details https://support.f5.com/csp/article/K9476. There are license check date requirements though. Check here for License-Check-Date/SF compatibility matrix https://support.f5.com/csp/article/K7727

You can renew support to raise your License-Check-Date but this can be costly. And even then, you'll only ever get to install BigIP 12.1.2 software and no later. Not sure if it's enough or not for your lab, you'll be the judge. Jumping from v9 to 12 is not a major hassle if you're happy with factory default configuration. If you run into issues, use BigIP 10.2.4 as your stepping stone. If 10.2.4 to v12.1.2 upgrade doesn't end well, use BigIP 11.4 as another stepping stone.

Also check this article by Chase Abbott regarding developer VE license (10Mbps) which suits perfectly for dev/test environments. Unlike with BigIP 3600 that limits you to 12.1.2 or older, you can use VE dev license with all current versions. https://devcentral.f5.com/articles/f5-developer-edition-how-to-obtain-a-developer-license-key