Is it true that you can play wow online with installing all three games

you might need to clarify this question a bit, but i will answer like i think you want.

you can play wow online if you install the client without the expansions, yes, but you will not have access to the updated content in either of the expansions. you will not be able to go to outlands or northrend. you will not be able to create a death knight in other words, just install all the expansions, its lots easier.