I have used some time trying to get DHCP/TFTP/NFS to work with the PI-3 (I do have PI-4s as well but that's not the issue).
Basically the Pi-3B+ is slow and flakey in a netboot situation and based on careful network analysis I think I know why:
The ARP responder doesn't work reliably.
So while the Pi starts up nicely and contacts the DHCP for info it is really a game whether it decides to reply to ARP requests at all. Sometimes it will answer for a time, sometimes not at all.
It will send the TFTP requests but leave it up to the Server to use a stale or failed arp entry, which it of course in the end won't stand for.
Any ideas around that?