2/3 channel home cinema

@heidrun  It is a much better case to include as much system information so that the respondents can offer you opinion with more accuracy. Again what is your budget? What is your goal?

A centre speaker takes the speech load from the side speakers when playing movies, for instance. It adds focus to that part of the speech score, while other speakers are available to take the music score and special effects.

1. I am assuming that a Sub-Woofer may be a plausible inclusion. This will fill in the lower notes and give you the visceral feeling in lieu of a full 5.1/7.2 etc system. There are some excellent Subs out there

2. There are some excellent soundbar/sub combinations (Yamaha/Samsung) that will lift your roof off.

There are very cheap ($$$ wise) amplifier/speaker combinations that can be considered as a typical 5.1 surround layout.

Remember that a good surround sound system and sound as good as a stereo system

Without bags of gold to purchase 2 systems (1) for Stereo only and, (2) for star wars big effects, you are going to need to plan your equipment  requirement carefully and moving through the second hand "bins" and forcing a bargain there. Good luck.

AMG

amg has some good points.  The audio for all movies are mixed for a minimum 5 channel system.  For movie dialogue, the sounds are mixed so that they are output primarily to the center channel speaker.  The left/right will produce ambient reverberation of the voices.  In other words, voices sourced from center will slightly echo out of the left/right.  This gives you an element of depth - like how somebody sounds talking in a small room or like a bathroom.  This adds to the reality of the movie. 

The surround speakers (in addition to providing special surround effects) will further produce environment reverberations.  If you don't want them, they are not really necessary, but will add some element of movie experience.

Of course, you can watch a movie with just 2 speakers (left/right), but the sound will not be as "solid" sounding and may even have some slight echo effects on the voices.

Music score is mostly output on the left/right main speakers and may reverberate some to the surround speakers.  All speakers can be used for special effects, such as explosions, car crashes, doors opening to the left, motorcycles riding from left to right, etc. etc.