Good Morning

I am a newbee to both your site and the plumbing I am about to try to do. I am building a new garage. By the way nice site you have and alot of help. I am wanting to install a 3/4 bath and tie into my existing sewage tank. I am just going to stub in the water, electric and sewage for now. As for the sewage, I was told I needed a 3 foot piece of 4" to stub the toilet, 2" for the shower/sink and 3 inch for the sewage line to the septic tank and 3" for the vent. I am going to put the bathroom in the south east cornor, with the layout being: Shower, toilet, and sink. Do I put the 4" for the toilet, then a 90reducer for the 3" main and then the "Y" for the shower and vent from the 3" or would the "Y" for the vent/shower go into the 4" straight piece. I hope I'm not confusing you guys. Appreciate your help guys.