y placement

I am installing a 3" line for a toilet and teeing of that with a 2" line for 2' of wetvent for the vanity and vent toilet. I know from reading post here i need to use a y fitting to connect to the 3" line on the horizonal. My ? is can I lie the y flat or does the y have to be upright ?

Y

A "Y" can be installed in just about any orientation.