The three dots on the thumbnail means you have placed a build on at least one item on that slide. A triangle in the lower right corner means you have placed a transition on that slide.
I would inspect the items on the slides with the three dots and see which ones have a build. You may have accidently had the Build window of the inspector (yellow dianmond, 3rd from left) open instead of the Transition window (slide icon, 2nd from left) when you entered a "movement" from the drop-down lists. You can easily cancel the builds by choosing "None" from the drop-down list.
I would inspect the items on the slides with the three dots and see which ones have a build. You may have accidently had the Build window of the inspector (yellow dianmond, 3rd from left) open instead of the Transition window (slide icon, 2nd from left) when you entered a "movement" from the drop-down lists. You can easily cancel the builds by choosing "None" from the drop-down list.