"She was born with three legs."
Translation:Narodila se se třema nohama.
We allow both S and SE in both of those sentences. the "SE SE" is the main translation here, as you can see above.
I would personally use either even for this sentence. If there were seven hand and seven legs though, it would be SE The entire purpose of S-SE is fluency of pronunciation. S SEDMI just makes you hiss too much so SE SEDMI it is.
Two limbs could be S or SE but ten limbs would always be only S because it is followed by DESET and saying SE DESETI just does not work.