Which IDE are most people using for mutliplatform projects? Will I have to use IDEA and CLion if I want to do Android, iOS, and web?

There is no universal solution for now. Android + Web is IDEA, K/N is only CLion, without direct iOS support unfortunately (but you have autocomplete of iOS platform API). Probably JB will release new IDE or improve existing one to support all platforms that Kotlin supports

IDEA. Hopefully Jetbrains comes around and either creates a CLion plugin for it or releases a new IDE.

Andrey Breslav already mentioned new IDE for Kotlin, that it on some early stage of development

Thanks for the info!