HI Flash_liam,
you don't mention what type of filter you have either, undergravel or canister ?
if gravel you should be fine, try not to disterb it to much.
if canister I would drain most of the water from it this will stop the bacteria from suffercaiting(*spelling)
you can get away with a 50% water change, as long as the water is up to temp it's just not recommended.
you can help your self by getting water containers 20lt + and fill them NOW, that way when you come to use them in 3 weeks the water will have aged enough for there not to be a issuse with cemicals in your water suppy and the dissolved gasses will have gone(little bubbles that form on the glass when just set up).
with your fish Bill's idea is fine how ever if your fish get chilled you may get white spot , keep an eye on them after the movefor a week or 2.
best of luck on the move