Can someone tell me why you shouldn't have a three legged toad/frog in the bedroom? They all say don't but don't say why!

The 3-legged frog, or money frog, is a Feng Shui symbol to attract wealth, and it's usually placed diagonally across from the front door, in the Wealth Area of your home, or near your desk in you office. The front door is called the Mouth of Chi where wealth enters your home, so when you place the frog to look toward the door it can "gulp up" the wealth chi for the house. Avoid placing the frog on the floor. A bedroom should be reserved for rest and romance, so it's not a good idea to place the frog there, and you should also avoid placing the frog in the kitchen. Read about finding the prosperity place in your home - www.fengshuiforreallife.com/Detailed/188.html

I understand but what can you do if you live in a room which combines as a bedroom and an office + creative centre?

The best solution would be to move the office area out of your bedroom into another room if possible.