So "if I could run" is a hypothetical case (#2). Therefore:
Si je pouvais courir.
The other two cases are:
Si je peux courir. -- If I can run
Si j'avais pu courir. -- If I could have run (If I had been able to run).
There are three levels of "probability" with si-clauses:
If I run. -- Si je cours. (Possible)
If I ran. -- Si je courais. (Hypothetical)
If I had run. -- Si j'avais couru. (Impossible, it already happened.)
This is pretty much parallel what you would use in English.
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