While it is true that it would be frustrating to have to cancel because of attendance, when you have only 3 people there it becomes a bit more difficult to plan a full session when people can't show.
And yes...we are no longer kids, but we all didn't start playing RPG's until Nov. of 2014, we don't have any kids of our own, and we planned all of our sessions for the same day every month to accommodate all of our on-calls, weekend hours, and stuff like that... Plus we like to get together for awhile once a month because this is usually the only time that all of us guys get to hang out as a full group. If we tried to run it any more often with shorter sessions, attendance would be even worse.
This is really the first time this happened in terms of attendance problems, and like I mentioned in my original post, we have 1 person who has had unexpected work conflicts when he wasn't supposed to be working, 1 person who had a couple family get-togethers come up, and then this other guy who has another obligation to attend to...but it all has come up very recently and we never had issues with our core group of 4 that we started out with.
One of the managers that I used to work with had a saying...he even had it printed as a banner hanging above his office door.
"Life happens."
If I understand correctly from your first post, you scheduled out the whole year's sessions at the top of the year. I don't know about your group, but for me, while I might say in January that those dates look good, I don't know what else might come up later on. I certainly understand that it can be disappointing and frustrating to have to reschedule or scrap a session, but...life happens. It seems to me that the most frustrating part for you is that these sorts of issues haven't come up in the group previously. Look at it from this perspective: in the past couple of years that the group has existed, the members haven't remained static outside of your game sessions...new and different responsibilities and obligations have surely entered members' lives. Life happens.
At the risk of offending, it also seems that you're taking the fact that life happens a bit personally. For example, referring to the reasons the initial player was going to miss/be late as "excuses" (and, hey...they may be excuses, but I'm the kind of person that gives the benefit of the doubt). Or, while saying your group is aware that they don't get XP if they're not there, seeking to look for other, punitive consequences for missing the session. (Just my impression, and it may just be a consequence of the silent, text-only medium of a message board.) Deep breath. Life happens.
With this new development, rather than making a unilateral decision, float it out to the group: "We'll only have 3/5 of the group this month. Would you prefer to forge ahead, reschedule; or cancel this month?" I think asking the players - the ones who will be directly affected - will be much more beneficial than asking us?