On solar, you must have a check valve before the solar "source" (normally between the filter and the 3 way valve), otherwise, when the pump shuts down the water coming back off the roof can flow back through your filter, back through the pump and into the pool. Especially messy if you have a DE filter (trust me on this one, personal experience!).
One more thing - the 3 way valve needs to be "leaky". They used to make 3 ways that didn't seal completely, now you (or the solar installer) normally drills a 1/2" hole in the valve (inside obviously) when it is closed to the solar side. This lets the input side of the solar drain when the pump shuts off (the GoldLine/Hayward valve and the installation instructions for the solar panels all had some instructions on this).