Hannah,
Thanks for asking your question to planDisney!
Without knowing the exact brand and model of the backpack, it is difficult to be 100% certain, but as long as you meet the cooler size specifications, you should be good. Here are some of the rules that you should consider when bringing a cooler into the parks.
- You cannot bring in alcoholic beverages
- You cannot bring in glass containers (except baby food jars)
- A bag cannot be larger than 24" long x 15" wide x 18" high (61 cm x 38 cm x 46 cm)
I looked at a model of such a 3-in-1 bag online as a frame of reference (the HANERDUN 3 in 1 Cooler Backpack). This one had a size that fits well within the Disney bag specs I mentioned above. However, there is one possible snag. Disney does have a rule against bringing in a folding chair. I don't expect that what you are likely bringing in fits into what they mean by that, but it will come down to who checks your bag on the way into a park. The goal of that rule, presumably, is to keep guests from clogging up the parks with traditional folding chairs just to watch a parade or fireworks, and your bag would hardly be an issue. But be aware that determining how a bag fits into that rule will be at the discretion of the security personnel that you encounter each day.
BONUS TIP
Your cooler cannot have open ice or liquid. You can have plastic bottled or canned drinks, of course, but Disney requires you to use freezer packs or bagged ice to cool your items.
BOTTOM LINE
I feel very confident that your bag, if it is within the size specifications, will be just fine. There is always the chance that an individual security cast member may have an issue with the stool feature, but I think it is within the spirit of what Disney allows and prohibits. If I were in your shoes I would still try to find another cooler bag, just so you did not have the start of a day ruined by having to return to your hotel to change bags, but it is up to you.
Please feel free to write me back directly if you have any follow-up questions.
Have a Magical Vacation!
Doug