You will know when to use CONF 3 or FULL and which autobrake by performing a landing performance calculation. As you become more experienced you will have a feel for it but should still perform a calculation to confirm.
Personally I would rather not do a CONF 3 on a steep approach. I also prefer CONF 3 to approach and land with.
Clearly outside of windshear conditions on an allowable runway it doesn't make sense to use a combination of CONF 3, MED autobrake and FULL reverse. Check your QRH for the different braking actions to see the credit you get per reverser used with different Config/ Autobrake combinations.
Other than that your OMs/ FCOM will have a prescriptive list as to when to use either config.
The best advice: ask your training department - I'm sure they'll be happy to go through it with you.
Last edited by WhyByFlier; 12th Jul 2013 at 07:57.