"no abortion except in the case of rape, incest, or risk to the mother's life" is not a coherent pro-life position to have.

obviously nothing pro-lifers say is coherent or rational, but this is especially weak.

the fetus/zygote/embryo/whatever is exactly the same as a fully-developed human according to pro-lifers. that's why they say they're against abortion: to them, it's exactly the same as murder and should be treated as such. most pro-lifers are christian as well, so abortion is not only murder but it's also sacrilegious, because they view the unborn (a very creepy descriptor, thanks) as miracles from god or whatever.

by not being against abortion in all circumstances, you're saying that there's situations where it's not only ok to murder someone, but it's also ok to spit in god's face. and that doesn't sound very pro-life, does it?

there's only a few reasons why someone would say they believe this:

A) they lean more neutral in the abortion debate and want to appease as many people as possible. if they're politicians, they're likely to pass an abortion ban with these exceptions because it feels like a reasonable compromise.

B) they lean more pro-life because it feels good, but haven't or don't want to think about the issue in-depth. maybe they're christian, maybe they're not, but they make up a large percentage of the population and aren't likely to oppose legislation passed by A or fight against the next group,

C) they are legitimately pro-life, they think all the consequences of an abortion ban are great, but they're trying to appear reasonable to get some legislation passed. the end goal is a total abortion ban, of course. if they're also in group A they'll pass it themself, but if they aren't they rely on those in groups A and B to do it instead.