Pentium III socket is completely different to Pentium 4 Socket (actually there were 3 different P4 sockets)
Laptop cpu's usually used different sockets to desktop cpu's
Googling got this http://www.ehow.com/list_6974757_compaq-presario-r3000-specs.html
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