You bought a 5d3 and you then bought a cheap mic with an iffy switch?
My thoughts are... I hope your friend isn't paying you.
My 2nd thoughts are... if you aren't that serious about video then hold your hands up now. If your pal is depending on your cheap mic and attitude towards kit for a record of his big day then he is probably going quite far out on a limb.
Pay cheap, pay twice. Except with a wedding, there are no re-shoots.
I would expect a wedding videographer to have a wireless lav on the groom for the service, a line in from the pa desk during the speeches, and a sepsrate camera top mic for safety and anything ambient.
This would go through a beachtek or a tascam (tascam also lets you record, avoiding rubbish pre-amps on camera) and this would also give you discrete channels to work with in editing.
If all of this means nothing to you, then you shouldn't be considering shooting the only video record of the wedding...
If its the candid stuff you want, the in amongst it, then great... as a record of the day...... hmmmm naw.