Unus, Duo, and Tres
Roman Numerals

Unus, una, unum is declined like an adjective like malus would be except in the genitive singular (unius) and dative singular (uni):

Case	Masc	Fem	Neu
Nominative	unus	una	unum
Genitive	unius	unius	unius
Dative	uni	uni	uni
Accusative	unum	unum	unum
Ablative	uno	uno	uno

Duo is declined as follows:

Case	Masc	Fem	Neu
Nominative	duo	duae	duo
Genitive	duorum	duarum	duorum
Dative	duobus	duabus	duobus
Accusative	duos	duas	duo
Ablative	duobus	duabus	duabus

Tres is a regular third declension adjective which has no singular (duh):

Case	Masc/Fem	Neu
Nominative	tres	tria
Genitive	trium	trium
Dative	tribus	tribus
Accusative	tres	tria
Ablative	tribus	tribus

Cardinal Numerals:
(except for unus, duo, and tres, cardinal numerals are indeclinable)
unus-a-um : one (I)
duo, duae, duo : two (II)
tres, tria : three (III)
quattuor : four (IV)
quinque : five (V)
sex : six (VI)
septem : seven (VII)
octo : eight (VIII)
novem : nine (IX)
decem : ten (X)
undecim : eleven (XI)
duodecim : twelve (XII)
viginti : twenty (XX)
centum : one hundred (C)
mille : one thousand (M)

Ordinal Numerals: primus-a-um : first
secundus-a-um : second
tertius-a-um : third
quartus-a-um : fourth
quintus-a-um : fifth
sextus-a-um : sixth
septimus-a-um : seventh
octavus-a-um : eighth
nonus-a-um : ninth
decimus-a-um : tenth