The Gemara gives a shiur of 10/11 trees out of 100 planted not bearing fruit as bieng the threshold for replanting them. The Gemara explains this in terms of the 11 trees being a whole new field to plant, not like tinofes in grain.

Would the same proportion, but fewer than 10 trees also require replanting. For example, 3 (i.e. one more than 2) out of 20, where the total number of trees to be planted is not the shiur of a field, but the proportion is the same?

It is only 11 trees which equal a signficant "field" unto itself. Therefore, 3 (out of 20) would not qualify, and the worker would only have to replant 1 tree. (This is based on the assumption that in less than 100 trees, the proportion of bad trees that the owner of the field must accept is still ten per cent. It is interesting to note that this is not mentioned in any of the Poskim.)

Y. Shaw

The wording of the Rashbam might imply, though, the 3 of 20 is the same as 11 from 100.

M. Kornfeld