I think he is either very ignorant/ill-informed about the basics of biology and pregnancy, or he is working to the "ideal" he has in his head about what he wants even though it doesn't fit with the reality of your situation - or he's just not committed to having children with you.
It might well be that he's simply not very well-informed or hasn't really thought through -
how long it potentially takes to conceive,
possible miscarriage(s)
then the time you carry a child,
then the recovery time after the first birth before you feel ready to try again (or the desire to leave a manageable age gap between two children)
then the time it takes to conceive the 2nd,
and go through the entire thing again
before you can try for a 3rd.
If you just wanted one child then waiting 4 years might be risky in terms of fertility, but it could work. But waiting 4 years and planning to have 3 children? To give enough time for 3 to be a realistic possibility you'd need to be cracking on with it by next year.
As an example of how things can pan out, and how it can all take longer than you expect -
My DH and me weren't 100% sure we wanted children until I was 35 and he was 41 (we met when I was 31, married when I was 32). We weren't able to try to conceive immediately due to work travel commitments abroad. We started trying when I was 36, luckily I got pregnant in the first month (you certainly can't assume it will be immediate /quick at any time, but especially not in your late 30's). So I was 37 when I had our first child. I had a difficult birth which caused lots of pelvic joint issues and meant walking and mobility was very difficult for many months. So we didn't start trying for the 2nd as early as I'd imagined we would. I conceived after 4 months but had a miscarriage. Luckily I conceived again very quickly and had my 2nd child aged 40. I developed a dangerous medical condition during pregnancy which made it stressful, but thankfully all turned out well and it was a much easier birth and recovery 2nd time around. We quite wanted a third child, but were aware we were very much running out of time and undecided about whether we were too old to be doing it again, So we just stopped "trying not to" to see if anything would happen. I did get pregnant again, within a few months, but I had an early miscarriage and so, at age 42, because of the miscarriages and my pregnancy/post-birth health issues and our ages, we decided that our baby-having days were done.
Whether it's taking longer to conceive than you hope/having miscarriages/post birth health issues, even if all goes well, the whole process is longer than the 9 months I imagine he has in his head.
I think you need to have a frank chat with him where you discuss the amount of time you realistically have.