A tiny moth with a wingspan of 5 to 6 mm. The adults are rather attractive, with brownish grey forewings each with three creamy patches.  The larva mine the leaves of Lombardy Poplar, Black Poplar and Hybrid Black-poplar. The mines are quite variable in shape, but usual begin with a narrow black central line of frass.  The larva sometimes continues feeding in a 'green island' in a fallen leaf

Adult Leafmine

Photograph mine with backlight, and note host species in comments

Areas where the larval foodplant is present.

They fly in May and again in August.

The larvae mine the leaves of Poplar (Populus), with variable characteristics. The mines are found in late June and July and from September to October.

Distributed throughout England, where it is fairly common except in the west; there are records from Scotland and Wales, but very few. In the Butterfly Conservation's Microlepidoptera Report 2011 this species was classified as common.

Increasingly well recorded in Leicestershire and Rutland in recent years, particularly from the leafmines on Poplar

http://www.leafmines.co.uk/html/Lepidoptera/S.trimaculella.htm

https://bladmineerders.nl/parasites/animalia/arthropoda/insecta/lepidoptera/monotrysia/nepticuloidea/nepticulidae/stigmella/stigmella-trimaculella/