Toilet /shower DMV

[RMirenzi]

My bathroom has a two foot 3" drain into the main 4" stack, this drains the toilet, the toilet is the highest fixture and vents into the main stack.
I am adding a shower,
I can vent the shower with a 1/1/2" existing vent about 3 feet from the toilet and within 1' of the shower trap.

Can I insert a 3-2-3 Y fitting between the the toilet and stack to serve as the shower drain or do I need to tap into the main stack after it goes vertical. This tap in will lie between the toilet and the stack, essentially before the toilet stack vent pipe.

Seattle codes.

Thank You
Bob

[Terry]

If you put a wye in the toilet trap arm, you will also need a 2" wye between the 3x2 wye and the p-trap. The 2" vent would then tie back into the 4" vent stack at 42" above the floor.

This essentially wet vents the toilet using the shower vent and prevents the toilet from siphoning the shower when flushed.

Seattle code