I would vent the three sink drains with a studor air admittance valve and vent your toilet through the vent or use a Studor AAV for each of the two sinks that are far away and the exisitng vent for the other sink and the toilet.
Studor AAV's are used every day with great success in the southeast US where I live.

http://www.studor.com/index2.htm

AAVs (Studors) are not listed under all the plumbing codes.

Every drain needs a vent (of some type) to prevent trap siphoning when water drains.

So does that mean I have to dig up the concrete around the toilet drain so I can attach the vent? I was hoping to avoid that.

Thanks for the info.

Every trap needs a vent, but not every vent is a separate line.

From the little information available I would suggest you extend each sink riser up to the ceiling, then connect all three to a common horizontal line tied into the vent pipe you mentioned.

If i had a complete floor plan with the dimensions I could probably find an easier solution but suffice it to say, if you follow the plan I mentioned above you will be fine.

Thanks - you had it right on the sink vents. I was planning on running one horizontal vent line and tying all 3 sinks to that vent line.

The toilet drain is still a puzzle to me. If I do need that drain vented (sems like the answer is yes), then do I have to jack hammer around the drain and connect a vent line to it? I could run that vent over to the horizontal line and thus have 3 sinks and the toilet all tied to the same vent.

The reason I ask is that in my last house when I had the basement bathroom roughed in they put in a sink drain, a toilet drain, and a vent. They plumbed the 2 drains so that the single vent line serviced both drains. And the vent line was already hooked up - although I had to move it a little when I framed the bathroom.

In the new house - they installed a toilet drain and a sink drain (a few feet apart) where the bathroom will be - but no vent connection. I believe that the sink drain is tied into the 4 inch toilet drain - and then the 4 inch pipe runs over to the main sewer line that dumps into the septic system.

I am wondering if I could attach a vent line to the sink drain and would that also provide venting for the toilet a few feet away? Then I won't have to dig up any concrete.

Thanks

A "Vent Line" is technically a dry pipe that solely conveys air from the atmosphere to a drain pipe. Code prohibits any horizontal "Vent Lines" until the vent reaches an elevation at least 6" higher than the flood level rim of the highest fixture served by that vent. The reasoning is simple, if we had a vent pipe, which is to say a pipe that is constantly dry running horizontal under the floor, whenever the drain line that it is attached to is conveying a liquid, some of that liquid, and by extension some of the solid particulates suspended in that liquid would back flow into the vent line. When the liquid stops back flowing into that line, then reverses direction and flows back down to the drain line the suspended solids could drop out of suspension and build up a deposit in that vent line, which would eventually block the line and render the vent useless.

The solution is to install a TEE on the drain riser behind a sink and connect the vent line to the top of the TEE. The line from the top of the TEE to atmosphere remains dry so it is technically defined as a "Vent" and in such times as there is no flow from the sink, that line remains open all the way to the horizontal drain lines under the floor, providing the necessary venting to the underfloor horizontal pipes but when the sink is used it discharges liquid into the riser and subsequently into the horizontal section under the floor which would wash away and solid particulates that might have recently accumulated in the horizontal section under the floor, thus the line from the Tee behind the sink fixture arm down to the horizontal drain lines under the floor is technically defined as a "Combined Waste & Vent" and is permitted because it is constantly washed from the sink discharge.

Under both the Uniform Plumbing Code(UPC) and the International Residential Code(IRC) all toilets with a flush greater than 1.6gpf are required to be attached to a 4" line, while all toilets that have a flush equal to 1.6gpf or less are required to be attached to a 3" line.

Under the UPC a 4" waste arm may run up to 10' from the fixture to a vent and a 3" line may run up to 6' from the fixture to a vent. Providing we have a combined waste and vent line attached to the toilet fixture arm within the prescribed limits mentioned above, we have proper venting from the combined waste & vent back to the toilet.

All pipe downstream from the point where the "combined waste & vent" attaches to the line it then defined as a "Vented Branch line" and may run an indefinite length.

Under the IRC a 3" waste arm may run 12' and a 4" or greater line may run 16' from the fixture trap to a vent however there is an exception in the IRC that says if a watercloset (toilet) is the only fixture served by that fixture arm, it may run an indefinite length from fixture to vent, vented branch drain or vented main drain.