These knives were issued for a number of different uses, AF pilots, army survival, engineers' utility, etc. Camillus and Ontario made them although Camillus is the name you see most often. The important dates as far as collectables are the Vietnam period. The one with the black on the handle has been altered, none were ever issued with black on the handle(there was a Navy knife that looks similar to the black handle but was nearly two inches longer). As far as a collectable, that makes it a no-go so that one is your best bet for a re-firb since you cannot harm the value now. The other two would bring in the neighborhood of $40-100, depending on the dates. We sell them in our store quite often and they usually go pretty fast if the condition is good and yours look good from just what I can see in the pics. The big question is do they have the original sheaths? That is key for any collector. It should be a sheath with a pocket on the front for a small rectangular sharpening stone. The better sheaths were made by Camillus and had a metal strip that wrapped around the bottom of the sheath from front to back to prevent the blade tip from punching through the leather.
I've seen several of these re-firbed in the past and was told this....(not that I'd do it though....I like the old rugged look of these knives like they are)....just grind off the peened down end on the tang and remove the cap and leather washers(easier said than done too!). The guard will slide off once that has been done, they usually are loose anyway if the knife has been used/abused heavily. Then coat the blade and re-assemble with new washers, just use a little less than there was to start with and you can put the cap back and peen down the tang. The knife ends up being a tad bit shorter in the handle but it's hardly noticeable if you do it right. If it sounds like a lot of work, IT IS!!! Especially for a knife that sells new for $69 right now today in almost any army-navy store in the country. They sold for $29.95 twenty five years ago when I first started in the business! LOL
Good Luck!