using vlookup tp check for criteria

I need to work out the follow using the vlookup table below

I need to convert levels to numerical values and then:

Firstly, add together two vlookup values THEN divide by 2 to get an average AND THEN see if this average AND a second, individual lookup value are above a specified another value, which may be different. IF all these criteria are set, return, "yes" if either the first or second, or both criteria are not met then "no"

Or put it another way

if lookup values A+B/2>"5" AND C>"3" then "yes", Else "no"

Lookup chart:

P1 1
P2 2
P3 3
P4 4
P5 5
P6 6
1c 7
1b 9
1a 11
2c 13
2b 15
2a 17
3c 19
3b 21
3a 23
4c 25
4b 27
4a 29
5c 31
5b 33
5a 35

Thanks