Bled is small, and that's the point. One lake, one island, one castle, and a path around it that takes about two hours. Most visitors see it on a day trip from Ljubljana, take the photo from the same two viewpoints and leave mid-afternoon. The 5 days in Ljubljana guide does exactly that day, and it works. This guide is for the other way: sleeping at the lake.
What you get by staying is the part the buses miss: the shore at sunrise, the lights after the tour groups leave, and time for the region around it. Bled itself is a lake, a promenade, a castle and a cream cake, and an honest verdict on the cake is below. The reason for three days is what's next door: Lake Bohinj, about 40 minutes away and much wilder, the Savica waterfall, Vogel above it, and a day of old town and bees in Radovljica. Two days feels rushed, and five repeats itself unless you hike or ski seriously.
Day one is the lake (viewpoints, island, castle, cake), day two a day trip to Bohinj, day three Radovljica, the Vintgar Gorge when it's open, and the lights at the lake. It's written for November to early spring, so each stop says what's closed. Start on a Wednesday: day three then falls on a Friday, when the Beekeeping Museum (closed Mondays) is open and the lakeside Winter Fairytale stays open late. More on the country is on the Slovenia page, and for skiing in the same valley see 3 days in Kranjska Gora.