when i connect, my isp is giving me 4MB. but when i use a VPN i get 10MB. xbox maybe uses a secure private connection like this and so your higher speed on xbox can be explained by this.
get some trial vpn and test it yourself, bet you go from 3MB to 10+MB
software speed trackers / isp speeds ... fake news !
isp's love to throttle your speed from the beginning, only after you complain will they fiddle with your baud and increase your speed (doing this can make your line unstable).