Which computer should I use for TS3?

If you want to play TS3 with all or most of its packs, especially Pets and Seasons, you really do need a dedicated graphics card and at least 6, or preferably 8, GB RAM, as well as a processor of sufficient strength. These requirements probably eliminate your old imac and would likely have eliminated your macbook air as well, had you tried. Still, it would be helpful to know the specs of each computer you're considering. For macs, here's the relevant info and how to find it:

https://answers.ea.com/t5/The-Sims-Mac/READ-FIRST-Sims-3-Mac-Help-Guides-and-Troubleshooting/m-p/4756624#M4

If it's easier, you can run a dxdiag on your HP laptop (or any Windows computer), post the results in Pastebin (no need for an account; just use the free option), and leave a link here.

https://answers.ea.com/t5/The-Sims-3/HOW-TO-DxDiag-PC-Only/m-p/1049584/thread-id/47181

It should be noted that most of the issues related to running TS3 on newer macs are entirely solveable. A very helpful mac expert (BluebellFlora) has put together a couple of guides to fixing the unknown error issue, as well as instructions on how to force recognition of incompatible graphics cards. I had little trouble installing and playing on my 2017 macbook pro once I found her website.

However, on macs, the game is limited to using 2 GB RAM due to the way it was ported into mac OS. This might be fine for your purposes, depending on your playstyle and how many packs you'd like to have installed, but it's almost impossible to run the entire game for any length of time without bumping up against the RAM limit. A few months ago, I was fed up enough to partition my hard drive and Bootcamp Windows 10 onto my mac for the sole purpose of playing TS3. It's a much better experience, and I would absolutely recommend it if you have a large enough hard drive and the willingness to deal with Windows 10.

Anyway, all of this is theoretical until we know the specs of your various computers and the packs you'd like to run.