The first and second are fine, with the first perhaps being slightly superior in that it hints at making exercise some kind of regular thing, while the second could suggest you’re recommending they do it only this one time.
The third risks some ambiguity, which the comments to your question noted, in that the plural “exercises” could refer to something other than physical activity; e.g. homework exercises.
Note that “go and exercise” is another option, and would actually be more idiomatic than “go exercise” in, at very least, Scotland; “go exercise” would be the more common of those two in the US.
Finally, as several comments have suggested, the phrase “by exerting extra effort” is making the statement (and your question about it!) a wee bit more complex than it might be, and you might even see it as redundant. Removing it from all your examples changes the meaning only marginally, but may make your overall intent much clearer.