We have 2 node 126.96.36.199 rac enabled database as primary and 1 node single instance database as standby database. I am planning to apply psu. I know that i have to apply psu on standby database first and then to primary database. So now as standby is single instance, i should apply database psu and for primary i should apply GI psu..is this the correct way of patching..or am i wrong..
Apply the GI PSU independently. It's a different set of software from a different Oracle Home so you can either do it before hand or afterwards or even at the same time if you want to. But of course the standby will be down temporarily while you do the GI patch (as it will stop the clusterware and ASM). So patch GI independently whenever you like.
Then for the DB PSU apply the software to both the primary and the target (OPatch Apply). But the catalog script changes (catbundle.sql psu apply) to the primary only - those changes will propagate through data guard to the standby. If you have MOS access there's a note on this there.