This is easier to do if you convert your matrix to a data frame using as.data.frame(). In that case the previous answers (using subset or m$three) will work, otherwise they will not.
To perform the operation on a matrix, you can define a column by name:
m[m[, "three"] == 11,]
Or by number:
m[m[,3] == 11,]
Note that if only one row matches, the result is an integer vector, not a matrix.