Turn off hyphenation?
Don't use KFX, which is less similar than azw3 to epub. The advantages of it are oversold.
That seems to me like intended behaviour, not a glitch because of three rather than 3.
Also 3 rather than three is more common for time. It might not hyphenate
Code:
started at 3 <span class="smallcaps">P.M.</span> and went
Chicago guide & AP has 3 p.m.
3 pm is also common.
Style guides also indicate when numbers should be digits and when spelled. Phone numbers, time, credit cards, house numbers etc should be digits.
Sometimes the space is the smaller space used for a value as digits with units as in 3 kg. Those also come in regular and non-breaking.
For compatibility only PDF or paper should use small caps and smaller size spaces.