〜としたら／とすれば／とすると
Supposing, if we assume (setting up a hypothesis)machine translation
FormationPlain form (verbs, adjectives; nouns/な-adjectives + だ) + としたら／とすれば／とすると
〜としたら means "supposing that…": the speaker provisionally treats A as true and then continues the discussion, even when A is very unlikely or contrary to fact: もし一億円あるとしたら (supposing you had a hundred million yen). Being a pure hypothesis, this pattern goes very well with もし and "what if" questions.
The three forms differ only in nuance. としたら is the most common and works freely in conversation. とすれば sounds rational, leaning toward deduction: 引っ越すとすれば、駅に近い場所がいい (if one were to move, somewhere near the station would be good). とすると usually opens onto an inevitable consequence, along the lines of "in that case it follows that…": 彼の話が本当だとすると、大変なことになる.
You're already familiar with the four conditional patterns from N4; what sets としたら apart is that it does NOT require the condition to be realizable — it merely "puts a hypothesis on the table" to think about. While なら latches onto what the other person just said, としたら is posed by the speaker themselves, which is why it works even for far-fetched scenarios like winning the lottery or turning back time.
Examples
もし一億円あるとしたら、何に使いますか。
Supposing you had a hundred million yen, what would you spend it on?
引っ越すとすれば、駅に近い場所がいいです。
If I were to move, I'd want somewhere close to the station.
彼の話が本当だとすると、大変なことになりますね。
If what he says is true, this is going to turn into something serious, isn't it?