Which brand is better? Nikon 1, sony nex and olympus pen?

I would like to buy a digital camera that can change the lens and the spec and picture quality are similar to DSLR but I don't know which brand should i buy.
at first, i want to buy DSLR but it's inconvenient if i want to use it for travel or bring it out.

For mirrorless, I would go sony

The Sony has the bigger sensor.

Don't buy into the, 'Full functionality with Minolta/Sony AF lenses,' though, the adapter which allows it costs more than most lenses.

Really all three brands are just fine.

Sony is really the newest kid on the block (making still cameras since 2005)

Both Nikon and Olympus have been making optics since the early 1930's and modern cameras since the 1940's, so they really have the track record and experience in that realm

You really can't go wrong with any of those cameras, but you need to do the research to see what the major differences are between them.

Do NOT forget to visit a proper camera store and hold each of those cameras in your hands to get the one that "feels" the best to you

As far as a dSLR being inconvenient, that is a matter of experience. Most of us who were used to using 35 mm SLR cameras carried them everywhere and changing to a digital body and using our old lenses was almost a seamless transition. I still carry a dSLR and four lenses in a backpack when I travel.