Sounds like clover. It creeps all over lawns and borders. It is a sign of nitrogen shortage in the lawn but it is quite a harmless weed. I normally just mow it out, though it is quite persistent.

Your only option is to use lawn weedkiller treatments, so next year, think about using one of the combined weed and feed treatments - these are used in April, and then again 6 weeks later. Alternatively, there's a liquid lawn weedkiller which you mix in a can and that's usually slightly more effective - Verdone is its name, but it does rather depend on the weed involved. Clover generally remains untouched by any weedkiller, but many lawn weeds put out 'threads'. Post a photo on here for ID

If it is clover, it would be a good idea to mix liquid weed killers with a surfactant, to help them stick to the leaves. Some brands have surfactants as part of the formula.