Okay, I looked up the rule and it says r are always tripped except

after n or l
or at the being of a word where it would be trilled

so please confirm enfermero is all tripped

If by "trip" you mean "roll", then no, none of the r's in enfermero are tripped.

A single spanish 'r' is more of a single "tap" of the tip of the tongue to the alveolar region or the roof of the mouth (the ridge right behind the upper teeth). It is similar to a 'tt' in English as in 'better'. A single 'r' at the beginning of a word is rolled.

The reason an 'r' after an l or n is somewhat rolled, is because the tip of the tongue is already in that position, so a roll facilitates the pronunciation.

There is never a spanish 'r' that sounds like an English 'r'.

Sorry if i am misinterpreting what you mean by trip.

¡Mucho gracias!

I am not sure why you are using "trip" here, but Mark's description is quite accurate: the single R is a single tap of the tongue (must be what you call "tripped"), and the thrilled one (the hard one to pronounce) has several vibrations (typically between 2 and 4, but it can have more if you maintain it longer). In "enfermero" they are all pronounced as single.