Finally!! Something I know about on this website. Well anyway I go to Aviation High school where we learn how to fix planes so I'm pretty familiar with the three you specified. So heres what I think:
Riveting- pop rivets (aka blind rivets/hand rivets) are good fasteners as long as theres no major structural or shear load on them(they will sheer off). they're easy to remove and install. Real rivets (which probably wont be used in FIRST very often) are excellent semi-permanent fasteners. they're reasonably easy to install and remove(though installation takes some practice and specialized tools) and they can take a good amount of sheer loads.
Bolting- bolting, and screws also, are good for anything you would want to remove constantly. my team used these the most this year on our robot since it was mostly a basic andymark everything. the good faster but they can get annoying when you have to use them in conjunction with moving parts.
Welding- lastly, theres welding. probably the best and most permanent (can something be most permanent?) fastener out there. just try removing a weld and see how far you get. downside is you have to get a little bit of training before you can properly weld. furthermore welding makes mechanical troubleshooting all the more difficult (depending on the application).
all that being said i would have to go with bolting most of the timesince its the best all-arounder.
