Once you've got the hang of what the declensions are about, it's much easier to learn the rest of them. However, the 3rd declension is not as easy as it seems, so take it with a pinch of salt. Good luck!
3rd Declension introduction
3rd declension is all genders (masculine, feminine and neuter), and its distinctive feature is that, although there are patterns, there is no set nominative. The neuter patterns are the same as the second declension; nom. and acc. are the same in the singular and plural (however nom. acc. plural end in 'a' -> 'please remember every day, neuter plurals end in 'a'!'
A few of these common nominative trends are; -tas -> -tatis -s -> -tis -x -> -cis/ -gis -ps -> -pis
Special rule alert!!
The general rule for the genitive plural (where it is -ium or -um) goes like so;
For civis, civis (same amount of syllables in NOM sing and GEN sing) = -ium
Where there are a different number of syllables in NOM sing and GEN sing = -um
Exceptions include; a) canis, iuvenes, senex, sedes, pater, mater, frater b) fur
3rd declension endings
Singular NOM
rex
VOC
rex
ACC
reg-em
GEN
reg-is
DAT
reg-i
ABL
reg-e
Plural NOM
reg-es
VOC
reg-es ACC
reg-es
GEN
reg- um
DAT
reg-ibus
ABL
reg-ibus
Important irregular nouns
Some very common irregular 3rd declension nouns are; -Iuppiter, Iovis (Jupiter), -bos, bovis (cow), vis (force/ strength), nemo (no-one).
Sg (both only have singular because Jupiter is a name, and nemo is a pronoun)
Singular NOM of 'Iuppiter'
Iuppiter
VOC
Iuppiter
ACC
Iov-em
GEN
Iov-is
DAT
Iov-i
ABL
Iov-e
Singular NOM of 'nemo'
nemo
VOC
nemo
ACC
neminem
GEN
nullius
DAT
nemini
ABL
nullo
The other two are; bos,bovis and vis
sg pl
NOM bos boves
ACC bovem boves
GEN bovis bovum
DAT bovi bobus
ABL bove bobus
NOM vis vires
ACC vim vires
GEN - virium
DAT - viribus
ABL vi viribus