。
Mother tied three pencils together with a single piece of string.
Literal
Mother [topic-は] 1 [counter-本] [gen-の] string [with-で] 3 [counter-本] [gen-の] pencils [obj-を] tied.
Both the string and the pencils use the counter 本 — both qualify as 'long thin objects.' The ~の between counter and noun is a prenominal pattern (数+counter+の+noun) that feels a bit written/careful; colloquial speech would often drop it or restructure as 紐１本で / 鉛筆を３本. で is instrumental here: 'by means of the string.' 結わえる is a fairly specific verb for tying or binding things together, a step more formal than 縛る.