Hi John,
1stB: 即使星期天,他也会每天锻炼身体。(place subject “他”in the second part of the sentence, because even there is a “即使”the main structure also follow by SVO, in the first part, “星期天”is a complement as unico, if it would be complete, it could be written : “即使是星期天” Predicate(which refer to verb)is “是” 。Then in the second part of sentence there is missing Subject. Where you could put “他”. The whole sentence structure would be “ the first clause (after 即使) is a supposition, which is then struck down as impossible by the second clause (after 也) of the statement.“
2nd B is okay. But it is better you put “即使”in front of Subject. Which would be “ 即使我很累,也会每天锻炼身体。(as well, the first clause it has S:我。V: 很累, for the second clause, because it has the same S, there is no necessary to repeat)
3rd B: 虽然他比我年轻(no 的很),我们仍然很高兴
first of all, the clause used after”即使”should be hypothetical, in your sentence, “he is younger than you” could be a truth. So in this case, “虽然”is being used here.
Second, “年轻得多” is okay,the structure “ Noun 1 + 比 + Noun 2 + Adj. + 很多 / 得多 / 多了” .
(Note: it is “得” 多。 not “的”多。 的+Noun. 美丽的花。得+adj: 高得多。地+Verb:认真地做。it’s better to check /review on this point in your study)
Third: the second clause in this case no need to put “也”, instead”仍然”(”still”would be better “)
Hope it would help you in some point.
Wish you all good.
Ciao.