When the notes are of the same valuation, but with different divisions. On unusual or rarely used triplets. Slurs are used when the note values are an eigth note or less. Many engravers do not use the slur, because they feel that the number alone is sufficient to indicate the triplet. If a legato slur is needed, it should be placed on the opposite side of the notes as the number, whether or not there is a triplet slur.
Some engravers place the number near the notehead end, others the stem end.
Some engravers put the numbers on top of the stem no matter which direction the stems are going. In either case, the number must be easy to see, so it should not fall within the staff lines. However, it may be necessary to place the number in the staff in some cases. If there is a conflict with a slur, put the number on the opposite side of the grouping as the slur. However, in some cases the number will have to be on he same side as the slur, because of divided voices on the same staff.