ive got a compression tester... however its most likely the valve has become unseated = head off job, reseated valves new H/G and cam belt while your there!
3rd valve sounds like an exhaust valve so you may notice a different exhaust note, rough running, rather shaky engine! etc etc.
the valve has prob dropped. so it needs it head off and the valve resating... i did the same job on a clio not so long ago + a full service for about £250/. i think
depending on the damage if its just valve 3 thats dropped then around £200 tops (i do full services mega cheap!). i send all heads away to be fixed by pro's so it comes back in a re con state. all nice and clean too thats with a new H/G, bolts, cam belt too.
I'm doing this today, the exhaust valve is buggered. You can text by putting a bit of oil on the exhaust manifold, and seeing which one doesn't burn off.