Welcome to the RingCentral Community

with this setup it will go to extension 3's voicemail (I strongly believe) It also matters what sort of call handling you have for extension 3. Why don't you test it yourself?

If you want it to go to extension 1's voicemail you would need to click on "Ring all together" and in the pop up that opens switch to "Ring in Order". In that same pop up, you will be able to group Extension 1 and Extension 2 in a Ring Group. and then below them add Extension 3.
Click "Done"
Then change "if no one answers" to my voicemail


The call flow is

INCOMING CALL => Ring Group (ext.1 and 2 simultaneously) => Ext 3 => voicemail

There are some extra tips I can go into - you can tell me if you are interested or if you got your answer already