By "broadleaf grass as well as low-growing brown flowers" you must be referring to a type of millet, and "brown clover with yellow flowers" sounds like oxalis. Oxalis can be dealt with using a universal lawn weedkiller.
The millet you describe is most likely to be hairy crabgrass (Digitaria sanguinalis), cockspur (Echinochloa crus-galli) or green foxtail (Setaria viridis) - based on your description it is not possible to be any more specific. Sweet grass is generally killed by frost but by then it will have sown thousands of seeds. As far as I am aware, sprays for dealing with millet in lawns are not permitted for use in residential or small gardens.
Millet mostly appears in patchy lawns and it is often found in sports turf and pitches ("Berlin Zoo grass seed" is often used) that are naturally less densely packed. I would therefore recommend that you seed and maintain your lawn so that the turf grows as thickly as possible. Doing so will hopefully prevent the millet from germinating. Ornamental lawn seed contains more types and varieties of grass that form runners, allowing these lawns to bind closer together than sports turf.
It is also possible to tackle millet in early summer, before it flowers and sows its seeds: As soon as it grows, fertilise the lawn heavily so that it grows rapidly. The millet will grow as well. Then mow the lawn and millet very short (2 cm). When wet, the grass will be flattened, unlike the millet. You will then need to re-sow the patchy lawn immediately so that it quickly becomes very dense again.