If a is your data.frame and you want to use a loop:
for(i in 1:nrow(a)){
a$results[i] <- ifelse( (a[i,1]>=3 & a[i,2]>=0) | (a[i,1] >=2 & a[i,2] >= 1) | ..., mean(a[i,1],a[i,2]), NA) # & is the AND | is the OR
}
The better way would be to define a function for the job and use apply to loop through the data.frame.
f <- function(x){
ifelse((x[1]>=3 & x[2]>=0) | (x[1] >=2 & x[2] >= 1) | ..., mean(x[1],x[2]), NA)
}
a$results <- apply(a,1,f)