You will probably need to pare down your list of desired places to see. If you don't already have your plane tickets I would start with open-jaw (multi-city) flights into Paris and home from Rome or Barcelona, depending on which route is more efficient. If you are locked into the round-trip to Paris you need to allow yourself a day to get back to Paris from either Spain or Italy. Your other option would be to land in Paris and immediately take a flight to either Rome or one of your Spain destinations and then work your way back to Paris and visit there before flying home.
Allow yourselves at least 4-5 full days in Paris, if you want to see Paris and take day trips to Giverny and Versailles, and 3-4 full days in Rome. Go through the rest of your list of places and decide how long you would need to spend there to satisfy you, then consider how many days you have remaining from your 28 and start eliminating if need be.
I've not been to Spain so I can't speak to that but the places you want to see in Italy would normally be a 2-3 week itinerary just for Italy. You might want to just do Paris with day trips (5 days); Venice, Florence & Rome in Italy (10-12 days); Barcelona, Seville & Madrid in Spain (8-9 days). Don't forget that travel distances and times between some of your destinations can be long so that will take up a bit of your time also.
A lot of your decisions will depend on each of the three woman's desires (must-sees) so it may be difficult to pick what to keep and what to drop. Also your sightseeing styles (museums, cafes, outdoor activities, etc) need to be taken into account.