In case you have a column that you want to separate e.g. the first column bellow:
and you want to make out of it three new columns
chr21 33032286 33032287 rs17881180
chr21 33032856 33032857 rs114905802
chr21 33032987 33032988 rs6650814
chr21 33033000 33033001 rs4816405
Select the first column
Go to Data/Text to column
Under Separated by type in the separators with the space between.
Before you do this make sure you have made two new columns in between, since the separation would overwrite already existing (2nd) column.