Hello!!
formula for Bohr'so radius is 0.59×n^2/z,where n is the number of orbit or the principal quantum number and z is the atomic number of atom,for He atomic number is 2 and since you are asking about 3rd orbit it would become, 0.5×(3)^2/2=2.25.
Thanks
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile