I will try to give you some ideas and plan what to see in Slovenia in 3 days. It will be quite busy plan so maybe you will not be able to see everything what I will suggest. Anyway I think it is possible to see all that but you will need a car instead of using public transport. Cause with public transport you will spend too much time for waiting and you can’t afford that if you come to Slovenia just for 3 days.
Start a day with visiting Bled. There is a very beautiful lake with an island and castle above the wall. First you can walk along the lake. There is nice path all around the lake so you like hiking you can go all around the lake. It takes about 1 hour, depending how fast you will walk. I suggest you to walk to the castle. From there is nice view on Lake Bled and its neighbourhood. But there is some admission fee to get into the castle. If you are not so much interesting in Castle museum, Wine Cellar and other buildings on Bled Castle but still want to get a wonderful view of Lake Bled you can go on Straza or Ojstrica. These are hills above Lake Bled. You will not need so much time to come on the top. At Straza you can also use chair lift.
After Lake Bled I suggest you to visit Vintgar Gorge. It is a beautiful gorge and it is just a few kilometers from Bled. In the narrow gorge you will walk on some kind of boardwalks along beautiful emerald river Radovna. Gorge is more than one kilometer long so take a good shoes cause at some parts could be slippery. At the end of gorge is Waterfall Sum and some snack bar where you can get something to eat and drink. Then you have to come back to the starting point by the same way.
From Vintgar Gorge you will be on the way to Bohinj. It takes about half hour of driving by car. Lake Bohinj is on the border of the Triglav National Park. It is a little less tourist than Lake Bled despite in high summer season can be very crowded too. When you come there you can walk a along the lake and enjoy in beauty at Ribcev Laz. After that I suggest you to visit Savica Waterfall. It is at the end of Bohinj valley. There is walking path to Savica Fall that takes about 20 minutes from parking area. On the way back you will drive to the lower cable car station that will take you to Vogel. It’s a mountain above Lake Bohinj and ski slope in winter. From there you will have a beautiful view on Lake Bohinj and Julian Alps. From the upper cable car station you can also hike higher if you will still have time. Spend the night at camp Zlatorog that is near lower cable car station if you like to be more in nature. Or find some room at some of hotels or at many private accommodation offers around.
From Bohinj you will drive to town Kamnik that is northern of Ljubljana. You can take a quick stop in Kamnik and walk around old town centre and up to Mali Grad. Then you will drive further to lower cable car station for Velika planina. Take an exciting ride where you will go high above the ground floor. At the top I suggest you to take also chair lift to spare some time. At the top you will be deserved with a beautiful view of Kamnik-Savinja Alps and on Velika planina with many shepard’s cottages. Walk around the cottages, you can visit chapel or some of the mountain huts around. Probably you will meet some cows around. You can get also some sour milk at some of the shepard’s cottages. On the way back I suggest you to take a quick look at Predaselj Gorge. It is just near the road, not so far away from lower cable car station for Velika planina. So you will not need more than 5 minutes to take a look at a very deep gorge from the bridge above.
From Predaselj Gorge go directly to Ljubljana, capital of Slovenia. In Ljubljana you must see charming old town centre, walk along Ljubljanica river, take a look around nice old buildings, etc. You can climb to the Ljubljana Castle or use funicular. From Ljubljana Castle is a wonderful view on Ljubljana and far neighbourhood, especially in nice weather. If you still have time you can walk from old town centre promenade to Tivoli Park or maybe visit some museum. In the evening you can go to Irish pub or some other bar. Spend the night in some of the hotels or hostel.
From Ljubljana you will drive in the Slovenia coast direction. On the way there I suggest you to visit Postojna Cave or Skocjan Caves. Both are worth to see but you have to choose just one because of the lack of time. Postojna Cave has an interesting train ride at the start beside its beauty of couse. Skocjan Caves has an interesting canyon inside the cave that also looks great and Skocjan Caves are a little less tourist than Postojna Cave. If you decide to visit Postojna Cave than I suggest you to drive to Predjama Castle and take at least a look at it cause probably there will not be time to visit whole. If you decide to visit Skocjan Caves then you can take a few kilometers Educational Skocjan Trail where you will see interesting nature and cultural heritage.
From the caves you will continue to Slovenia coast and visit old mediterranean town Piran. There is one of the most beautiful squares in Slovenia, Tartini Square. All around are nice old buildings and Tartini Monument. From there you can go uphill on narrow streets to Church of St George or Town Walls. From both are beautiful view on Piran and cliffs. At the end of busy day you can sit down at some bar and drink a beer.