First off, don’t be intimidated; it’s easy.
Second, the original copy of this miniature guide of mine is on the third page of the ‘Casa’ thread.
The regular rules for the third declension are:
(Note: to recognize a third declension noun, you must look at the genitive singular, not the nominative, because sometimes the nominative sg. of a noun resembles that of a noun of another declension. i.e., scelus (neuter) - sin, wrongdoing, evil)
Regular MASCULINE/FEMININE nouns of the 3rd declension
Nom (whatever) -es
Gen -is -um
Dat -i -ibus
Acc -em -es
Abl -e -ibus
e.g.
Virtus, Virtutis - Virtue/Bravery/Masculinity
Virtus Virtutes
Virtutis Virtutum
Virtuti Virtutibus
Virtutem Virtutes
Virtute Virtutibus
Regular NEUTER nouns of the 3rd declension
Nom -(whatever) -a
Gen -is -um
Dat -i -ibus
Acc (same as the nom.) -(same as nom pl.)
Abl -e -ibus
Scelus, Sceleris
Scelus Scelera
Sceleris Scelerum
Sceleri Sceleribus
Scelus Scelera
Scelere Sceleribus
-I stem nouns (-ium nouns) of the 3rd declension:
There are special 3rd declension nouns, which end with -ium in the genitive plural case. Here are the rules for those:
If a feminine or masculine noun has the same number of syllables in the nominative singular as in the genitive singular, the noun is an I-stem noun.
Nubes (nom), Nubis(gen) Cloud
Nubes Nubes
Nubis Nubium
Nubi Nubibus
Nubem Nubes
Nube Nubibus
CANIS (dog) and IUVENIS (youth) are two exceptions to this rule.
If a feminine or masculine noun ends with -s or -x in the nominative sg. and has two consonants in its stem, it ends with -ium in the gen. plural. Note: you do not often see the stem in the nominative.
Arx, Arcis (see the two bolded consonants which comprise the stem?)
Arx Arces
Arcis Arcium
Arci Arcibus
Arcem Arces
Arce Arcibus
However…
Rex, Regis - King, Ruler
Notice that the stem does not have two consonants!
Rex Reges
Regis Regum
Regi Regibus
Regem Reges
Rege Regibus
NEUTER I-STEM NOUNS
Any third declenion neuter noun, ending with -al, -ar, or -e, will have a different ablative singular, accusative singular, nominative pl, and a different accusative pl., and, also, it will have -ium in the gen. pl.
Animal, Animalis
Animal Animalia
Animalis Animalium
Animali Animalibus
Animal Animalia
Animali Animalibus
Mare, Maris
Mare Maria
Maris Marium
Mari Maribus
Mare Maria
Mari Maribus
Summarized rules for I-stem nouns of the third declension
Masculine and Feminine Nouns
Nom (blah) -es
Gen -is -ium
Dat -i -ibus
Acc -em -es
Abl -e -ibus
Neuter Nouns
Nom (-al, -ar, or -e) -ia
Gen -is -ium
Dat -i -ibus
Acc -(same as nom.) -(same as nom. pl.)
Abl -i -ibus
I hope this helps!
Wow! Thanks! That’s incredible…I’ll definitely use it! And it is helpful!
Thanks!
Keesa
Keesa third declension is like on page 100 of D’Ooge 
It will take quite a while to get up to it, concentrate on what you’re doing now as each page is so important that you must not be distracted.
It hurt me when I was; the passive stopped me for a while.