In today's modern notation: XVI-XI-MCMXXXVIII
LXV
Letters are not numbers. Not all letters in the alphabet represent Roman numerals though some letters do.
XXV-VI
Just use the letters I V X L C D and M
CCCLXXVIII
Using Roman Numerals: VI/IX/MMXI uses a total of 4 letters.
MCMLXXVII
MMX
IX
DCXLI
You could use roman numerals but then you would need 6 letters: XIII-V-XI.
3210 = MMMCCX in Roman numerals which are expressed in letters