In regards to the multi-pills: I think your decision should be based on your location and human comfort. I live in a very humid area of NC (USA) with extremely mild (often 80s) winters. Each "Momma Tick" can lay 30,000 eggs, "Momma Flea" can lay several thousand over her lifetime. And "Momma Mosquito" only needs a Tablespoon of water to lay her eggs in. Huge families of them survive in my area year round. And I don't want them in my home. So, for (admittedly) completely selfish reasons, doodle gets 'Simparica Trio' every 6 weeks.
If you live in an area with extremely cold winters, you may not have issues with fleas. Ticks can typically be found with a good inspection when the dogs come inside. Mosquito sightings may be few and far between. You might not need something that repels all three... but, someone else might live in an area where more protection is prudent.