- A reference guide and manual in Russian for Heroes 3.

I disagree. I think it always rounds down. For example, until it reaches 2.0, it rounds down to 1.0. Between 2.0 and 3.0, it rounds down to two... If I believe your words, then 2.0 will never be reached at all. When transitioning from 1.8 to 2.2, the rounding, according to your post, will be upwards - to 3.0. Or am I not quite understanding your example? Explain it then.

No. If the bonus is 1.8 at the current level, and when the next level is reached, for which a bonus is due, it will be 2.2, then at the current level, the bonus is rounded down to 2.
For example:
Tamika with a Black Knight.
Level 6: +1 attack/defense
Level 12: bonus = 18 * 0.05 = 0.9 - rounded down to 1.
Level 18: bonus = 18 * 0.1 = 1.8 - rounded down to 2.
Level 24: bonus = 18 * 0.15 = 2.7 - rounded down to 3.
And Galtran with a General Skeleton.
Level 1: +1 attack/defense
Level 2: bonus = 6 * 0.05 = 0.3 - rounded down to 0.
Level 3: bonus = 6 * 0.1 = 0.6 - rounded down to 0.
Level 4: bonus = 6 * 0.15 = 0.9 - rounded down to 1.
Level 5: bonus = 6 * 0.2 = 1.2 - rounded down to 1.
Level 6: bonus = 6 * 0.25 = 1.5 - rounded down to 1.
Level 7: bonus = 6 * 0.3 = 1.8 - rounded down to 2.
Level 8: bonus = 6 * 0.35 = 2.1 - rounded down to 2.
Level 9: bonus = 6 * 0.4 = 2.4 - rounded down to 2.
Level 10: bonus = 6 * 0.45 = 2.7 - rounded down to 2.
Level 11: bonus = 6 * 0.5 = 3.00 - rounded down to 3.
Level 12: bonus = 6 * 0.55 = 3.3 - rounded down to 3.