3 weeks in an open top primary fermenter is a *long* time to have oxygen exposure.
Taste it ... does it taste/smell like vinegar? ... if it is infected, I’d think an acetobacter infection would be a hell of a lot more likely.
Acetobacter needs oxygen ... eliminating oxygen contact and headspace is critical in such an infection.
So I’d take an SG and pH reading and rack to a carboy, eliminate headspace and put on an airlock. With all that exposure, even tho it’s cider I’d add a dose of sulfites too if I thought it might be infected.
Not likely the growth of a brett pellicle ... pretty rare in most home winemaking ...... assuming you’ve *not* used brett or had it before. (I assume that’s what you are asking.)
But I suppose if it smells like a barnyard it could be brettanomyces.
*Not* that it’s likely, but as far as Brett goes ...
Chill the must down to 35* to 38* for a week, this should kill it as brettanomyces (and dekkera) is cold temperature sensitive. Then rack from the middle as you suggested, into another bucket ... add a light dose of k-meta to the cider ... and chill at 35* for another week. Finish the ferment with a yeast with good competitive factor (K1-V1116 comes to mind).
If it is brett ... anything that came in contact with it that was wood has to go. For instance, when you get brett in a barrel you might as well smoke a ham with it. (or give it to someone who makes sours or uses brett for adding “character” to certain wines.)
A good thorough wash with detergent/PBW and sanitizing with Starsan etc should be fine for non-porous equipment although I might consider replacing any flexible plastic (siphon) tubing that came in contact (in the case of brett).
If your cider must has no off odors or flavors it may be fine ... but at least get it away from oxygen and under airlock like I noted above.