I think that estar is a tricky verb for the example as I've said in the comments.
I agree with fedorqui on sentence #1
Si pudieras vivir donde quisieras, ¿Dónde vivirías?
In sentences #2 and #3 your problem is that you are looking for concordance in the subordinate sentence (the one introduced by que), and while you certainly cannot use any tense, there is no requirement for concordance.
Let's rephrase sentence #2 without estar, which IMHO shows it more clearly:
Si vendieran el coche, lo compraría.
The concordance is vendieran (imperfect subjunctive)/compraría(conditional indicative).
But in this sentence:
Si vendieran el coche que vi en Madrid, lo compraría.
But you can replace vi for a number of tenses: he visto, había visto, veo, veré... They all work, although they convey a slightly different meaning. Similarly you can replace the subordinate sentence by other complements:
Si vendieran el coche de Madrid, lo compraría.
Si vendieran el coche amarillo, lo compraría.
Can't resist to mention the name of the song by Les Luthiers:
Quien conociera a María, amaría a María.
The same could be said about sentence #3