A good utility for writing the images is [http://www.seasip.demon.co.uk/Unix/LibDsk/#tools John Elliot's dsktool].
A guide to connecting a 3" drive to a PC and reading/writing disks is available from the [http://www.worldofspectrum.org/sdp/FAQ.html
|Spectrum Disk Preservation Project]
====Writing individual files====
To write individual files to a +3 formatted diskette (or dsk file) you can use [http://www.cpctech.org.uk/download/cpcxfs.zip cpcxfs]