I actually did this exact project a few years ago. However I ran a smaller 5v water pump, light sensor, and moisture sensor, and an LCD screen. Depending how you plan on operating your plant system, I would recommend having your uno plugged into a portable charger, then having your solar panel wired to charge the power supply during the day.
If you wanna keep it basic then you should get a single, larger panel that can supply enough power to those and its less complicated than wiring 3 separate solar panels (plus higher risk of one failing) since the Arduino takes 5v to power, plus the 9v for the pump so that's 14v needed to run your watering system. If you'd like I can send you my project files and you can edit or use them how you please.