I thought the batteries would power more than those 3 instruments mentioned! (Capt Fathom)
I think that you are correct there. Acc my manual the main standby bus and the apu standby bus remain powered by their respective batteries for at least 30 mins. The main standby bus powers (among other things) the left EIU, left CDU, left ILS, left VOR, various flight control components, left ADC, left EFIS control panel, and the primary EICAS display.