The Compaq Presario R3000 utilizes a single-core, 1.6 GHz Athlon 3000+ processor and 256MB of DDR SDRAM. The Athlon processor uses less energy than Intel processors, lending the three-hour battery life of the Compaq laptop.
about only reasonable upgrade would be more RAM
sometimes there are similar model names for AMD & Intel versions
Device Manager will show current CPU type
CPU-z will show more details
Yes you will find the pins on a P3 vs a P4 and the socket to take them simply do not match.
So your sort of out of luck there, in order to do it, it would mean having to buy a new board. And if you think about it for the hassle you better off buying a new Cpu and the board to go with it, giving you more upgrade options. I think it`s time you though about doing a serious upgrade of the components it would be worth far more to you to go that route, in the long run don`t you think. Think of the plus side to doing it.
Faster Cpu, more memory, larger storage. better connectivity, if a laptop.
Even a P4 cpu wont make it much more better for running things today.