P0303 references a misfire on #3 cylinder. Ignition faults including spark plugs , cables , distributor cap and rotor are common problems.
P0440 is a malfunction in the evap system. Items to check include gas cap (is it loose?) , evap purge valve ( leaking or blocked internally) and plumbing/hoses to fuel tank.
Any place fuel vapors can escape is basically involved.
There are specific devices known as "evap testers" or "smoke testers" to find small leaks.