There has never been any difference in the support span before, and the release notes mention none for Kubuntu 18.04. So, for the lack of counter evidence, it is safe to assume that both 32 and 64bit packages will get 3 years of support.
The 'main' archive of Ubuntu 18.04 LTS will be supported for 5 years until April 2023. Ubuntu 18.04 LTS will be supported for 5 years for Ubuntu Desktop, Ubuntu Server, and Ubuntu Core. Ubuntu Studio 18.04 will be supported for 9 months. All other flavors will be supported for 3 years.
Yesterday, developer Dimitri John Ledkov wrote a message on the Ubuntu Mailing list calling for the end of i386 support by Ubuntu 18.10. Ledkov argues that more software is being developed with 64-bit support. He is also concerned that it will be difficult to provide security support for the aging i386 architecture.