Why would phase 1 be the most common phase for sex? From what I understand it’s a very short period of infertility (like a few days)?
Not the most common - I think you misunderstood what I said.
Phase 3 is naturally the most common. Once ovulation is detected and passed, that is the safest phase for relations. Couples who want to be very risk averse (say for serious health reasons) may choose this more conservative approach.
But most couples (as far as I know) choose to use the (usually brief) infertile phase 1 too. It is a more inherently risky phase, but those familiar with their cycles and fertility symptoms may be comfortable in using this phase. Otherwise, the restriction of relations can be very long. Some people, who have irregular cycles, may enter phase 3 quite rarely, and thus restricting to phase 3 only can be very stressful on a married couple who wish to have relations.
Understanding your cycle, across all phases, will give you the best information to decide on the risk level you wish to take, and weigh this against your reasons for avoiding pregnancy, noting that the reasons and the acceptable risk level may very well change over time.
Conversely, if you do not properly understand phase 1, you will be restricted to phase 3 only, and you may over time find this to put great stress on the two of you, depending on your desires for relations (which may be unequal between any given couple).