What are your BG levels ?
The diet for someone with Type 1 should be the same as someone without diabetes. The only reason why Type 1 is likely to cause weight loss is if your BG is high.
If it is not the case, then eating more should help with weight gain. Just make sure you increase your insulin accordingly.
If it does not, I recommend making an appointment with your GP as weight loss may be indicative of some other health problem. Three stone loss without trying is concerning.