From Space ALC:
http://home.alc.co.jp/db/owa/jpn_npa?stage=2&sn=121
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QUESTION:
What's the difference between "お金があると旅行します", "お金があったら旅行します", and "お金があれば旅行します"?
ANSWER:
..."〜ば" and "〜たら" can be used to represent what might happen if a certain non-extant state comes to be; they can represent a hypothetical situation. "〜と" cannot.
..."お金があったら旅行します" and "お金があれば旅行します" may infer "Because [I] don't have the money, [I] won't travel". "お金があると旅行します" does not imply this.
...When "〜ば" and "〜たら" are used in this hypothetical fashion, "〜ば" represents a general hypothesis (based on assumed universal cause-and-effect relationships), and "〜たら" represents a specific hypothesis (based on one's own particular situation).
For example:
"ホームレスであれば、盗人になる" = "If I were homeless, I would become a thief (because that's what the homeless do)."
"ホームレスであったら,盗人になる" = "If I were homeless, I would become a thief (in my personal case)."
..."〜ば" and "〜と" can be used to represent a habitual (i.e. traditional or repetitive) relationship between two actions, "〜たら" cannot.
For example:
"メキシコの水を飲めば、下痢をする” = "You get the runs if you drink the water in Mexico."
"メキシコの水を飲むと、下痢をする” = "You get the runs if you drink the water in Mexico."
So, the translation of each of the sentences in question are:
「お金があると旅行します。」
*"They (always) travel when they can afford it."
「お金があったら旅行します。」
*"They will travel if they can afford it."/"They would travel if they could afford it."
「お金があれば旅行します。」
*"They (always) travel when they can afford it." (so I know that:)
*"They will travel if they can afford it."/"They would travel if they could afford it."
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Just to throw in my own two-cents here, I'll explain the difference between other forms and 〜なら. 〜なら differs from the rest of them in that the order of the actions is reversed.
For example:
海外へ行くなら、医師に診てもらう。
"If I go overseas, I will see a doctor (before I go)."
海外へ行ったら、医師に診てもらう。
"If/When I go overseas, I will see a doctor (after I get there)."