I found a cool manual to install the 3i on a usb flash drive.

source: http://communities.vmware.com/message/825686

Requirements:

• 1 GB USB flash drive
• WinImage
• IZArc

Of course you can use your favourite tools and larger USB flash drives, but 1 GB is enough space for the boot image.
Perform the following steps to create a bootable ESX Server 3i USB flash drive:

1. Download ESX Server 3i Installable ISO
2. Extract INSTALL.TGZ from the root directory of the ISO image using an IZArc
3. Extract /usr/lib/vmware/installer/VMware-VMvisor-big-3.5.0-67921.i386.dd.bz2 from INSTALL.TGZ using IZArc
4. Extract VMware-VMvisor-big-3.5.0-67921.i386.dd from VMware-VMvisor-big-3.5.0-67921.i386.dd.bz2 using IZArc
5. Attach the USB flash drive and make sure you no longer need the data on it
6. Use WinImage to transfer VMware-VMvisor-big-3.5.0-67921.i386.dd to the USB flash drive
7. Disk->Restore Virtual Hard Disk image on physical drive…
8. Select the USB flash drive (Warning: If you select the wrong disk you will lose data!)
9. Select the image file VMware-VMvisor-big-3.5.0-67921.i386.dd
10. Confirm the warning message
11. Wait for the transfer to complete
12. Unplug the USB flash drive (Warning: If you forget to unplug the flash drive from the PC you might lose the data on your hard drives the next time you boot!)
13. Attach the USB flash drive to the machine you want to boot (Warning: If ESX Server 3i recognizes local drives, you might lose the data on it, so make sure you don´t need it anymore or unplug all hard drives!)
14. Turn the machine on and make sure the USB flash drive is selected as boot device
15. Watch ESX Server 3i boot
16. Configure
17. Enjoy!

I cannot be held responsible for any damage or loss of data this procedure might cause.