The
First Steps Trilogy, released as an introduction to Pathfinder Society are free and are available at the top of the page
here. They each take a little longer to run than 3 hours, however,
Pirst Steps 2: To Delve the Dungeon Deep is the shortest of the three if you have the
Haunted Dungeon flip-mat to assist you in running the adventure.
We Be Goblins and
Master of the Fallen Fortress, the pdfs of which are both free on the Paizo site, might be useful to you and provide more "fun" than the
First Steps Trilogy as a one shot adventiure, imo. They will each require at least one encounter to be cut from them -- at a minimum -- to be playable in 2 hours, but you might manage either in three hours. On the plus side, they are free and FUN. If 3 hours is okay? Then I recommend either of these modules as your best bet.
PFS scenarios are a great choice. The scenarios in Season 0 and early in Season 1 tend to be shorter than those presented on late Season 1 and Seasons 2 and 3. Most of them will take at least 3.5 hours to play, however.
There are "delves" that are available to run in 60 minutes slots for PFS as well -- though I find those to be of indifferent quality for the most part. I would not recommend them.
If you really only have 2 hours, you can look into a "Quest" published in
Kobold Quarterly. Quests are specifically designed to be played in 90-120 minutes. The only one so far is the short adventure purblished in KQ #17 referred to above by Ironwolf.
Lastly, looking back in the annals of
Dungeon, you might also have a look at converting one of the "Side Treks" from the issues of that magazine which were featured in the late 2E era.
If lack of regular and recurring attendance to be able keep up a coherent story from session to session is you main issue (together with scenario length),
then PFS is ideally suited for your group. There are a number of home groups which run PFS for
precisely this reason. I know of several players in such a home PFS campaign played for this reason and it works very well for them and their needs.
If this is not a one shot but you would like to keep up with PFS as the context in which a series of one shots are run to provide an overall campaign narrative, Pathfinder Society is the
ideal solution for your group's needs.