[dirtymeat80]

at the start, you only get those 3 slots, so if you have tons in your pack, but only brought 1 with you during the select screen, you're out of luck.

Edit: as for the toolbelt, you can carry a few with you, and when you need the tool, look at the bottom left of the crafting UI, and you'll see a button for swapping the toolbelt.....click it, then click the toolbelt you wanna use