I recently bought a computer just for Platinum. I am having issues with songs I recorded on X1 X2 and X3 with samples missing synths missing etc. Do I have to redownload all the old software? What is the best way to solve these problems as Sonar is always changing?

My older X downloads are available in my store account.
Did you look in your account?

Any plugins referenced in your X1/2/3 projects need to be installed and recognized on your current system with Platinum.  That's just how virtual instruments/FX work. 
 
Audio is different.  If you had, for example, bounced a project down to a stereo audio wave file, then that file would play in Platinum without any need for the plugins that were in the original project.
 
Does that make sense?

Do I have to download X1, X2 and X3 or is there a better way.
 

Jon is correct. Any plugin that was exclusive to X1-X3 can only be installed through the X installers. So yes you may have to d/l the old installers to get the plugs you need.  

You will need to download the previous Sonar installers to get the plugins that no longer ship with Platinum.  Most of the plugins can then be installed without installing the full X1 - X3 program.  Just select the advanced install option and select the plugins that you want to install and leave everything else unchecked.
 
The exception to this is that I think you will need to install X2 if you wish to recover Roland V-Vocal.  I think that was part of the core install.  But for the others, just run the plugin installer.

A note about V-Vocal: It will work on SPlat but for some reason it doesn't display properly in the Multidock and needs to be in its own window to work right. Maybe one more reason its not included.

CANNIMAGINE
Do I have to download X1, X2 and X3 or is there a better way.
 

If you don't have the installation files yourself you'll have to download them. They'll be available in your online account. All versions of Sonar can be installed on the same computer as they are placed into their own folder under the C:\Program Files\Cakewalk\Sonar Platinum folder for example.
Install the oldest one first as items in shared folders get over written by newer versions. When you double click your installer file at some point you will be given the opportunity to select which plugins you want to install. Unless it's a plugin that's no longer available and you want to install it don't select plugins you already have as they'll be over writing current plugins with older versions.

To make sure you have all bases covered, once you're finished installing the older versions reinstall your current version of Sonar to make sure that any shared files are the most recent. A basic Sonar installation doesn't take long to complete.

All versions, X3 and before are NOT shown in the Cakewalk Command Centre nor do you use the CCC to do the installation. Hope that helps.