1) take the lower of the three percentages for individual subjects.
This is the more forgivable of the two mistakes. If the percentages meeting the expected standard in reading, writing and maths are 69%, 72% and 77% then you take the lowest one (69%) as the combined figure. I get why people do this and sometimes this is correct but don't assume it is. It is certainly the case that your combined figure cannot be higher than the lowest of the three, but it can be a lot lower.
2) take the average of the three percentages for individual subjects.
This is a common mistake and one that needs to stop. You don't average percentages. Well, there are occasions when it could work but this really isn't one of them.
Essentially, the combined figure is about Venn diagrams, the pupils that plot in the intersect of the three circles. Imagine you had 9 pupils and 3 pupils (33%) achieved the expected standard in each subject. Using the above methods your combined figure would be 33% RWM. But it could be 0% if we had discrete groups achieving the expected standard in each subject. The pupils that achieved it in maths, did not achieve it in reading and writing; those that achieved it in reading did not do so in maths and writing and so on. No pupils achieved all three and so none plot in the interect of the Venn diagram.
Imagine you are standing out on the playground on a winter's day and you note that some children are wearing hats, some are wearing gloves and some are wearing scarves. Some are wearing just one of the items, some are wearing two, and some are wearing all three. You would not calculate the percentage wearing all three items - hats, gloves and scarves - by working out the percentage wearing the individual items and averaging it. That would be nuts. You would simply count how many pupils were wearing all three items together. You could even draw a Venn diagram on the playground and get pupils to stand in the right parts of the circles.
That is no different to our approach to calculating %EXS in RWM. You need the pupil level data: a spreadsheet with the names of the pupils and 3 columns for reading, writing and maths. Then just enter a Y or N according to whether or not they met the expected standard in each subject. When you're done, count the number of pupils that have a Y in all 3 columns and divide that figure by the total number of pupils to get the correct percentage.
Or if you're feeling flash, write a clever =IF formula in excel to do it all for you. I do love an =IF formula.
Hopefully that all makes some sort of sense.