For the first question, you should replace the appropriate parts of the command you pasted, so the TR gets 3.000, prefix is followed by the name you want for the output, and the input volumes are the names of your EPI dataset volume files (maybe *.nii where the asterisk is a wildcard character).
The approximate TR will probably work unless you have a thousand TRs, and then the later TRs could be off by a second. Still it doesn't really hurt to set the TR exactly. The slice timing can be set with the 3dTcat command by adding "-tpattern alt+z" to get interleaved slice timing.