Emphasizing the important words: Each delver can roll
once per trip (in plain English, that means one roll in total; if we meant "once per skill per trip," we'd have written that). Each delver may choose one of Fishing, Naturalist,
or Survival for this roll. It's up to the GM to define "trip," which will vary from adventure to adventure and from land to land (and from GM to GM, of course).
So why learn more than one of these skills? For the things they cover
besides general foraging:
Fishing: "On voyages by boat or ship, Fishing is the only skill allowed for [foraging] – no Naturalist or Survival!" and "[make] a DX-based roll anytime it's necessary to cast a hooked or weighted line to fetch or pull something."
Naturalist: "A successful skill roll will let you [...] identify valuable animal parts or recall the capabilities of strange beasts" and "When in doubt about natural dangers, the GM will roll against Naturalist to see what you know."
Survival: "Survival rolls can lower [...] living expenses by letting you camp outside town," "The GM may permit Survival rolls to detect and avoid suitable mundane dangers," "When resisting hardships such as heat and cold, HT-based Survival can replace HT," and "remove [...] external parts from dead creatures."
Each skill has uses that go beyond foraging. These include
delving uses, like fishing for treasure, identifying monsters and their valuable body parts, and collecting said body parts.