File extension s3p is used by following file type(s):

File extension s3p file type 1 description:
AKAI MESA II/PC S-series program file

Files created by Akai MESA, software for S-series AKAI samplers.

that open s3p files:
Awave Studio (Microsoft Windows)


File extension s3p file type 2 description:
Touchstone 9 s-parameters data file

The S4P file extension is associated with the Touchstone File Format. S4P Touchstone file is an ASCII text file that contains information about n-port network parameter data and noise data of linear active devices, passive filters, passive devices, or interconnect networks.

The Touchstone file format was introduced in 1984 with Touchstone eponymous frequency-domain linear circuit simulator from EEsof (now Agilent Technologies).

that open s3p files:
gedit (Linux)
Microsoft Windows NotePad (Microsoft Windows)
NotePad++ text editor (Microsoft Windows)
TextEdit (Apple OS X)