The truck (DAF LF55 220, Year of Manufacture 2009 has some oil leaks near the exhaust manifold and during starting the engine, there are some blue smoke coming. 3litres oil was topped up after around 2000km recent journey. What might the problem
Not DAF specific but generally:
Oil on the outside of the engine? When cold, steam clean it, and see where the leaks are.
Oil consumption, and blue smoke? I would think about pulling the air hoses off the turbo, and checking for excess oil there. If you can feel any play on the turbo vanes it is ready to be replaced before it lets go. If no play apparent doesn’t mean it is OK, but will need a proper check, not a rough’n’ready one.
Other causes might be worn bores, or oil dropping past valve guides, but check turbo first.