Second, I will state a couple of rules that I believe I understand. My question should be a simple FRCP question, and then, based on that, I will ask how it relates to a specific FRCP issue that I am having, which appears to mirror a FRCP as well.
So there will be two questions that I would appreciate so much to have answered.

Here we go.
FIRST ISSUE THAT I HAVE
I believe I understand the FRCP to allow a Defendant ("D) 21 days after receiving the complaint and summons in his hands to serve an answer on the plaintiff's ("P") complaint.
Further, I believe D, in serving his answer on the P, is judged by a different rule. According to FRCP 5(a)(1)(B), "a pleading filed after the original complaint" must be served on every party. And according to FRCP 5(b)(2)(C), a paper is deemed served under Rule 5 through a litany of ways, one of which is..."mailing it to the person's last known address—in which event service is complete upon mailing;"
Therefore, D will be deemed to have served P after D simply mails it out, regardless of the date P actually receives it. In other words, if D mailed it out on the 21st day (the day the answer is due to be served), then D is fine. Even if P received the answer on the 25th day.
So that is the first issue I want to confirm that I have correct.
SECOND ISSUE THAT I HAVE
FRCP 6(d) provides in pertinent part, "When a party may or must act within a specified time after being served and service is made under Rule 5(b)(2)(C) (mail), (D) (leaving with the clerk) . . . 3 days are added after the period would otherwise expire under Rule 6(a).
I am having trouble understanding the logic of that rule. That rule suggests that if D was served by a federal marshal, then D will not have 3 additional days. However, if D was served by mail, then D receives 3 additional days. Why would D be given MORE TIME when D's time to answer is ONLY TRIGGERED AFTER being served.