The class 3SUB were DC suburban electric multiple units introduced by the LSWR in 1915 and introduced by the Southern Railway in the period up to 1939. The class designation 3SUB was not used by the Southern Railway, although some authors refer to these units as 3SUB. When rebuilt to four cars in the 1940s, they became class 4SUB.

Eighty-four electric multiple units, numbered E1–E84, were constructed in 1913 by converting steam-hauled carriages which had been built from 1904 for suburban service. The conversions being carried out at Eastleigh Works. Each multiple unit comprised two motor coaches and a trailer.