倘若, 万一, 一旦 — three kinds of supposing

規則 倘若 / 假如 = if — the written 如果, entirely neutral: 倘若有机会，我一定去。
万一 = in case, on the off chance — supposing something BAD and UNLIKELY: 万一下雨怎么办？
一旦 = once — supposing something bound to happen, with the weight on the consequence: 一旦决定了，就不要后悔。
万一 also works as a noun (以防万一); 一旦 and 倘若 do not.