The gardens are simply beautiful. The transport is on first come first served basis so get to the Adam tower early. The parking spot is indeed close to the Keukenhof entrance. The 3 hours interval is a bit tight but if you miss the bus scheduled and there are places available you can catch a later bus.
We had a fabulous day, lots of people, but to be expected. The weather had been cold leading up to our visit so some tulips were not at their fullest but the large tulip greenhouse was spectacular with loads of varieties. We loved it, coach drivers and staff all lovely.
It was a fun trip, as we had a short ride to get the tickets on the ferry boat. Then you walk to office to redeem voucher for tickets. Best to do this early to get a good time. We spent 2 hours there and saw lots, some may wish longer. Bus ride back to drop off point was easy and about an hour ride. Back on the ferry and then we caught a tram to explore some more of the city.
Bus trip out to Keukenhof was fine. Met at the A'Dam meeting area, loaded the bus and off we went. About half full on the bus but is was March 27 so not a huge crowd headed to the gardens. Short trip as advertised. Short walk from parking lot. Keukenhof is huge. I can see there will be really large crowds when the outdoor tulips bloom. There were some outside but thousands in the buildings. Hundreds of colors and styles. More than you can ever imagine. Took a lot of pictures. There were plenty of bathrooms and eating/drinking stops along the route. A very beautiful place. I would say a must see when in Amsterdam. The Viator tour is the way to go. Print out voucher, get on the bus, view the tulips. A Great Day in the Netherlands.