I drove to Kep two weeks ago and Route 3 is in the middle of major upgrades, so avoid it at all costs (esp the PP city end).

Recommended route;

South out of the city to Takhmau, down R2 to just outside Takeo, turn right onto Road 22 (clearly signposted) and travel to R3.

Turn left onto R3, but stay on R3 for only approx 6 km and take a left in the fork onto Road 31 (which turns into 33), towards Kep, avoiding Kampot. Google maps has a clear layout.

R31/33 is a smooth and wide road, well paved and fast (or slower and enjoy the scenery), will take you to the White Horse roundabout and into Kep.

We stayed at Champey Inn, just along from crab market. Pretty bungalows, good infinity pool, very friendly staff (French owner now manages the place), slow service - but that seems to be the norm down there. Breakfast was served on the terrace in front of the sea.