The on board weapons load-out for a Bf 109 G-2 was a single MG 151/20 mm cannon firing through the prop hub. In addition there were 2 x MG 17 7.92 mm machine guns in cowling.
2 x MG 151/20 mm under wing gondolas could be mounted as well.
See the link I posted above. This image shows the on board weapons configuration for a Bf 109 G-6 - as such the 2 cowling mounted machine guns are now MG 131 13 mm .
So if Weißenberger's 100th Abschuß was 'with 3 x MG 151s' then at first glance this would imply his Bf 109 G-2 was fitted with the MG 151/20 mm under wing gondolas. If his Bf 109 G-2 was not fitted with gondolas then there must be an error. It could mean he achieved this kill with his 3 'on board weapons' - i.e. 1 x Mg 151/20 mm cannon firing through the hub and his 2 x Mg 17 7.92 mm machine guns mounted in the cowl.
Good Luck,
Bruce