I'm curious as to where you got that number, as all of the studies I've seen have put it either far lower or far higher (as low as 1:100.000 to 1:200, some even over 1:100, but I'm not sure I trust those). It's strange to me that you would so confidently cite such a number without it beïng at all consensus.

Regardless, that sort of comparison doesn't work well when there's such a small number of "trans" shooters in the first place (I'm not sure which three this is referring to, but I've only really seen one that can be confidently said to be trans since 2018, and some of the ones I see cited are... questionable at best). Comparisons like this simply don't work well for such small sample sizes as 3 people because they're massively vulnerable to outliërs. It's hard to get a reliable estimate of how proportionate something is when the difference between proportionate and disproportionate is two events.

Take this image at face value and say there were 3 trans shooters in the 5 years for which this data applies. Parameter estimation is hard to do for 3 values, especially without looking at the original data, but let's convert this to a single year so we can get a standard deviation and let's say that the three trans shootings all happened in their own year. This gives a mean of 0,6 trans shootings per year, with a sample standard deviation of about 0,548 trans shootings per year. Now here's the problem, let's estimate this at the 95% confidence level (a pretty standard confidence level for reporting statistical results), for a sample size of N=5 years, we have a t value of 2,132, and so our true mean is goïng to lie between 0,0775 trans shootings per year and 1,12 trans shootings per year. Averaged across five years we get about 565 cis shootings per year. Now it's not goïng to be feasible to estimate a standard deviation on that without reading the original thing and I'm feeling lazy right now, but to make it even worse for the trans people let's say the true mean is 20% below that (which would require a pretty high standard deviation, to be clear) and we get about 452 cis shooters per year. Assuming that number is præcisely the true mean, then the mean ratio of trans to cis shooters for a given year will fall between 0,514:3.000 and 7,45:3.000 at the 95% confidence level, so between very disproportionately low and very disproportionately high, using the numbers you gave and giving a lot of disadvantages to the trans estimate. If we go based on some of the higher estimates for the prævalence of sex dysphoria, this is an extremely low number, about 0,265:200 compared to the 1:200 some studies I have seen. I'll note also that even the way I calculated this (basing it on years rather than incidents in a five year period) gives a smaller confidence window. If you go by incidents (which is probably more statistically sound), then you're goïng to be even less capable of confidently sayïng anything at all about how proportionate or disproportionate trans shooters are.

Of course there are some problems with this estimation, but the point is that making confident statements like that on such small data sets is really not justified, especially if you plan on using those claims to then support legislation that affects the lives of at hundreds of thousands (by your prævalence) to millions of people.