The PU is busy with R3. Make the call the best you can. If you aren't sure you may do two things. Either call safe or bang the out on timing. Either way it's your call to make. Make it and stick to it.
With a runner on third only you are probably in "B". From there move straight at the 45 ft line to get the best look you can. If there is too much complaining tell them to hire U3 and you will get a better look.