Three (3) modes which can be availed of without court intervention

There are six (6) different modes of discovery available under the Rules of Court. They are: [1] Depositions pending action (Rule 23); [2] Depositions before action or pending appeal (Rule 24); [3] Interrogatories to parties (Rule 25); [4] Admission by adverse party (Rule 26); [5] Production or inspection of documents and things (Rule 27); and [6] Physical and mental examination of persons (Rule 28).
Among the above, there are three (3) modes which can be availed of without court intervention. What modes of discovery may be availed of without leave of court and, generally, without court intervention?

[1] Depositions (whether by oral examination or written interrogatories) under Rule 24. This is before action or pending appeal;
[2] Interrogatories to parties under Rule 25; and
[3] Requests for admission under Rule 26.