Are you looking for synths specifically? You could load up on synth VST's all day, the real question is, which do you need and which are just wasting space on your computer? The 3 synths you listed there are plenty to do nearly anything you could ask for as far as sound design. Are you designing sounds or using presets? What kind of music are you making? Obviously there are very specific things that you may need other tools to accomplish but I can vouch for Massive and Sylenth as they are a few of my favorites. If you like the NI stuff, Absynth is a serious synth and FM8 is also nice. But really though, what are you looking for? What kinds of sounds, what kind of synthesis? More misc. recommendations would be Zebra2, Diva, Vanguard, KORG Legacy Bundle... could go on forever. If you provide specifically what you are looking for I can give you better answers.