For the X5, leaving apart other BMW models:

The 3.0 used from 2000 until 2006 was the M54 engine. 225 hp.

The 3.0 used from 2007 onwards was the N52 engine. 265 hp. It gained Valvetronic (variable valve timing). The cars were labelled with various badges, either i, si, or 30 depending on year and which model it was. All the same engine and power rating.

The 3.0 used from 2011 onwards is the N55 engine. 300 hp, due to the addition of a single twin-scroll turbocharger. This is badged as a 35. It is not a 3.5 litre, the stretch is due to BMW wanting to position it above the other (naturally aspirated) 3.0 engines, which are called 30.

None of them are V6 engines, they are all inline 6 engines.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue