The answer to the question you asked is easy -- yes, you can get an ERAS token and apply. It's very late for this year, so you'd be looking at next year or getting an ERAS application together for this year's SOAP.
But, the question you really want the answer to is: "How successful will I be when I apply to a new field after completing a PGY-3 in a different field and being unable to pass the board exam?" That question is much more complicated to answer.
In general, if you're switching because of a change of interest AND your performance in your current residency program is fine, it's possible to find a spot in a new field if you would have been competitive for that field in the first place. You haven't mentioned what field you're in but your post history suggests Anesthesia - and if so you're probably referring to the BASIC exam which happens at the end of PGY-3. This is important -- in IM the exam comes only after you finish all training, and even if you never pass it you can still work in some sites without it. But if this is an exam that comes in the middle of training (like anesthesia, or the physics exam in Rads), then if you can't pass it you won't get promoted and won't finish residency. So, big difference.
So, if you try to switch to a less competitive field, you'll have some chance of getting a spot. You may have funding problems since Medicare funding is capped, but this is sometimes not an issue. If you or your program says the reason you are changing is inability to pass an exam, programs may worry about that as we all are assessed on how our grads do on their board exams.
Your best first step, if you want to consider a different field, is to meet with the PD of that field at your home institution (if it exists). You might be able to get some credit for your prior training. It will all depend on the details.