Please help. I want to use a laptop with x3 playlist to play live with backing tracks but it wont import mp3s?

The play list supports sonar projects.  If you want to use it first import the MP3 into a project and save It as a cwp. 
 
If you need to use audio and midi synced in your backing, Sonars playlist is a great solution.  If you just want to play MP3's there are many simpler ways to play them back using a play list outside of Sonar. 

I like Playlist because of the space bar start and stop. I want to import an mp3 backing track and add tracks to it and convert to mp3 to make it more reliable on playback. Thank you.

It is a great solution for live backing tracks David.  I have been using it in my show for many years.   Once you save your backing tracks as a cakewalk project you really do not need to convert it back to MP3 though.  Just play the project from the playlist. The only preparation I do between arranging and recording my backing tracks and live performance is to bounce any vst synth tracks to audio tracks.  This speeds up the loading and ensures completely reliable playback.  It has never failed me on stage in 20 years. You also have the flexibility of fully editable multi track backing rather than just a stereo MP3. You also have the ability to run lights, guitar patch changes etc via midi. 

Latency scares me with tracks. Any skip or bump in the drive thru playback is horrible. What else should I know ...and thank u soon much

Latency is not an issue for playback.  You can set your asio settings  as high as you need it to be so that your system is completely stable.  It only comes into play when you are using midi to trigger vst synths or real time fx on inputs whilst recording tracks,  as you would notice the delay between playing a note and having it sound when latency is above about 12ms. Once you have all of your tracks recorded and are just playing them back you will not even notice any latency,  so you can set your asio audio playback settings with higher,  safer buffers. This way you will never need worry about glitches on playback. 

Is converting to 16 bit help or hurt for playback?

I find 16 bit absolutely fine for backing tracks.  They do place slightly less load on hdd playback.  That said,  given  the speed of modern PC's and hard drives,  the difference in system and disk load between 16 and 24 bit playback is negligible and probably not a concern outside of projects with very large track counts.