Hi Suzie and welcome.
It sounds like you have had a bit of a rough ride with your health to say the least. I hope you are starting to feel better.
@eggyg is our resident 3c poster girl but she is currently on holiday on Arran so may not pick up on this until she gets back.
Have you had surgery on your pancreas or non invasive cancer therapy (if that is the appropriate term)? Just wondering how much of it you have left and what functions it is able to perform? I know some Type 3cs need Creon to help digest food as well as insulin. Is that the case for you or perhaps you don't need insulin yet or maybe neither?
I very much doubt Metformin would be helpful to you as it mostly helps with insulin resistance (IR) and having been underweight by the sound of it, you are unlikely to have problems with that unless all the weight you have managed to put back on is fat.
If you can let us know what your HbA1c reading is and what if any medication you have been given for it that would help. If you don't have a BG meter for home testing, most of us find that an invaluable tool to help us manage our blood glucose on a meal by meal basis. It will show you which foods your body can cope with and which you need to reduce portion size or cut out. Generally it is carbs which cause us diabetics problems. It doesn't matter too much if they are sugars or starches, our bodies are pretty good at breaking them all down into glucose, so there is the obvious sweet stuff like sugar and cakes and biscuits and sweets but also bread and pasta and rice and potatoes and breakfast cereals. Even otherwise healthy foods like fruit in all it's forms (fresh dried frozen or juiced particularly) and porridge cause some of us problems and need to be restricted. It can be quite individual as to what and how much of it each of us can manage without spiking out BG levels too high and that is where the meter comes into it's own. By testing immediately before and then 2 hours after a meal we can see how our body coped with the carbs in that meal. If the post meal reading is more than 3 whole mmols higher than the pre meal reading, we ate too many carbs and need to look at reducing portion size.
Meters and particularly the test strips for them vary enormously in price but the two basic meters most often used and recommended by members of this forum for economy of use and reliability are the SD Gluco Navii or the Spirit Healthcare Tee2. Both are available online.
Think I have probably overwhelmed you with too much info already but hang in there, it will all start to make sense eventually. Any questions, ask away. We will be only too happy to help you.