Can you mix

[Sinster]

CAT 3 and CaT 5 Network cables? I have like 25 800' CAT 3 laying around.

 

[LeeJend]

1). Not if you expect a CAT 5 system.

2). How do you plan to "mix them".
-If you mean splicing CAT 5 and CAT 3 together to form one long cable, not a good idea. It will cause impedance mismatch and reflections that raises the noise level.
-If you mean using different types between different systems/HUBs you're back to answer 1). You will have no better than a CAT 3 system. So 10 mbps will be fine, 100 mbps might work and anything above 100 mbps is out of the question.

If you do use the CAT 3 keep them on the shorter runs and away from machinery, portable phone stations and wifi transmitters.