For 3 feet, you won't see any noticeable difference by upgrading cable, not worth the money if you already have what you need. Just use quality LMR-400, have the connectors installed and waterproof all your outdoor connections.
Rereading this, I should probably clarify....
RG-58 is fine for short jumpers. Yes, it's a lossy cable at higher frequencies, but the difference in losses between RG-58 and similar size/lower loss cables for such a short run isn't something you'd notice.
The important part is to have cable that is flexible enough that it doesn't put any strain on the connectors. Busting a connector off the scanner will not be worth it.
If you were starting from zero and needed all new cables, something like LMR-195 has a bit less loss, but is still flexible enough to not strain the connectors. Getting 3 foot sections of LMR-195 with new connectors installed would not be cheap, and it's unlikely you'd notice the difference.
In other words, use what you've got unless you really need to waste some money or want the bragging rights. Unless you have a really, really, really weak signal, you won't hear the difference, and even then, it would be a very minor improvement.
Upgrading the cable to your antenna is a good plan. Lower loss cable gets more signal down to your distribution amp. That's usually a good investment.
Do your best to avoid adapters. Use the correct connectors to mate directly to the equipment. Adapters add unnecessary loss and strain.
