Data + Power connector and cable suggestion
I'd suggest that RJ-45 and Cat-5 cable is perfectly fine: it's cheap and easy, cables available ready-made in any colour or shape you like without any difficulty.  The main downside is risk of connecting to ethernet (wouldn't damage anything, but would it be okay in your environment?), and worth thinking about the power.

At 3 metres and 500 mA you can work around the power issues:
Cat 5 is about 0.2&Omega;/metre, call it 1&Omega; for your cable Vdrop ~ 0.5 V.  I've found it very beneficial to have a small local power module eg  Recom R-78E5.0-0.5 and send the supply at 24V.  They are very cheap and made all my power problems disappear.  In your circumstance it would mean you could change to eg 6 metre cable without any issue at all.  I'd certainly much rather have local power than a bigger connector or custom cables.