My first project - creating a time lapse video

You don't want to re-compress your original video. All you need to do is import the video into After Effects, select the file in the Project Panel and use File>Interpret Footage>Main to change the frame rate of the video. A little math will get you exactly where you want to go. If the original is 1:20:00  and you want the final to be 3:00 you have the ratio you need to get the multiplier. An hour and twenty minutes is 80 minutes or 4800 seconds. Three minutes is 180 seconds. The ration is 26.666666666666667 to 1. Multiply the frame rate by 26.666666666666667 and you get 800. Your hour and 20-minute video will now take exactly 3 minutes to playback. Create a new comp and you are done. 

The other option is to create a new comp from the footage, enable time remapping, set the current-time indicator to 3 minutes and 1 frame, then cut the last time remapping keyframe and paste it to the three minute and one frame mark and trim your comp to 3 minutes.

The other option is to use the {} icon at the bottom left of the Timeline to view the In-Out Duration Stretch panel and simply double click the duration for the footage layer and set the time to 3 minutes, then trim the comp to the out point of the time stretched layer.

That's how I would handle the project.