>>>I removed the wall in the basement and saw that the 3" piping just goes across and down into the concrete floor. No p-trap!<<<<
Just goes across what?, is this your main sewer pipe?
>>>>I suppose all I need to do is saw at two places and insert a 3" diameter p-trap<<<<
Why do you think you need a 3in. trap?
>>>>- how close/far away from the washer can I put the p-trap (4 feet down + 4 feet across = 8 feet, is that ok?)<<<<
Have you been draining your wash machine down into the line?
I really can't get a grasp on what you have, can you post a picture?
The only thing I can state for sure is that you have to cap the line thats in the garage, it's tied into a live sewer line, major health hazard.