Are you working with AWS D1.1?
The welder can qualify in any position without having to qualify a different position first, there are no prerequisites.
Now if you are asking if the welder qualifying in a 2F position is also qualified to weld in a 3G position, the answer is no.
AWS D1.1:2015 Table 4.10. A test taken in 2F qualifies for Flat and Horz fillets only with plate, pipe, and box tube.
To get qualified for 3G the welder will need to take a test in 3G.