100g raw rhubarb is 4.5g of carbs...so not awful lot. It is difficult to give you exact answer as it would have been easier if you would have weighed the rhubarb before cooking to get the 'per portion' quantity. But as the carb content is relatively small, I would not worry about it and just work it out with the plain sugar...round it up to next full number and you are not that far off.
I don't have any sugar in house so I cannot work out what 3 teaspoons of sugar might weigh...so I leave to other to work it out for you unless you can do it yourself.
I would
questimate you portion is not far off from 7-8 g of carbs...
Edit to add...teaspoon of sugar is 4g of carbs...so 3x4= 12...that makes 3g per 100g portion....+ 100g raw rhubarb 4.5 =7.5g and because there is more carbs in cooked portion...hmm..maybe about 8.5g in total?
Under 10g anyway!