When performing Gayatri Japam, in the last term “dhiyO yO naH pracOdayAt” is it ‘naH’ or ‘na? Does ‘naH’ (Naha) refer to ‘Na’ (No)? Also, when performing Ashtakshara Japam 28 times, should Pranayama be done 3 times or 3 x 28 times?

dhiyO yO naH ‘ is a term. ‘naH’ means ‘ours’ (i.e., ‘our intellect’). When combined, ‘na: + pracO:’ = naph pracO, with ‘ph’ occurring in the middle.
It is correct to perform Pranayama 3 x 28 times during Ashtakshara Japam. Pranayama itself (in this context) means 28 repetitions. For Ashtakshara Japam, there are 25 or 28 counts. You can do 3 x 28. If that is not possible, one can easily do 1 x 28.