Should first-timers visit Siem Reap, Phnom Penh and the coast in one trip?
Yes, if you have enough time and accept the transfers. Siem Reap and Phnom Penh give the cultural and historical spine; the coast changes the mood, but it should not be squeezed in just to make the map look complete.
Answer
The three-part Cambodia trip can be excellent: Angkor first, Phnom Penh for recent history and city reality, then Kampot, Kep or the islands for a different pace. The catch is that each part asks for something different from the traveller. Angkor takes heat and temple stamina. Phnom Penh takes emotional energy. The coast takes transport tolerance and season awareness. With ten days or more, the route can breathe. With a week, one part usually needs to go.