How pretty! Yes, the purple flowers on top are petunias. I forget the names of the other two but they require the same care.
Petunias are sun plants, but in a pot like that are probably better off in part shade so they don't over heat. Does your pot have drainage holes? If so, watering thoroughly until the water comes out the bottom is the appropriate amount of water. (Without drainage holes, the plants would quickly drown.)
Typically, you would water when the soil begins to dry. But since you have trouble getting to the soil you can also water when the plants just barely start to droop. (Not lots of droop, too dry and you'll loose the flowers.) Fortunately, if the pot has drainage it is hard to over-water a petunia. Dependent on your weather and the pot, you may be watering anywhere from once a week to once a day.
They will be fine in the rain unless the expected rain is so heavy that it will flatten the plants . If you have frost expected, or hail, then they are better moved into the garage.
These are annuals, so should bloom all summer and then die when it gets cold in the fall. Cut off the dead flowers as they go crispy to encourage it to keep flowering.