Answer grounded in published legislation
Your question
According to namibian law is man aged 3 legal to have sex with some girlfriend aged 18 years + 6 monthes
The supplied provisions do not establish that sexual activity between a 3-year-old child and an 18-year-old is lawful. They state that the former rule presuming a boy under 14 incapable of sexual intercourse does not apply, and that a child accused of a sexual offence has criminal capacity assessed as a factual question in the same way as for other offences. 1
Section 14 cited here criminalises sexual or indecent acts with a child under 16 where the other person is more than three years older. An 18-year-old is more than three years older than a 3-year-old, but the supplied extracts do not provide enough detail to determine every possible offence or outcome. 2
Verified sources
Where this answer comes from
Combating of Rape Act 8 of 2000
has had sexual intercourse or has performed an act of a sexual nature with another person or is the father of any child, such question shall be determined as a question of fact, and no presumption or rule of law to the effect that a boy under the age of fourteen years is incapable of sexual intercourse, shall operate. (2) The criminal capacity of an accused under the age of fourteen years who is charged with an offence of a sexual nature shall be determined in the same manner as the criminal capacity of an accused under the age of fourteen years who is charged with any other offence. Abolition of cautionary rule relating to offences of a sexual or indecent nature
Combating of Immoral Practices Act 21 of 1980
(a) commits or attempts to commit a sexual act with a child under the age of sixteen years; or (b) commits or attempts to commit an indecent or immoral act with such a child; or (c) solicits or entices such a child to the commission of a sexual act or an indecent or immoral act, and who - (i) is more than three years older than such a child; shall be guilty of an offence and liable on conviction to a fine not exceeding N$40 000 or to imprisonment for a period not exceeding ten years or to both such fine and such imprisonment. [Section 14 is substituted by Act 7 of 2000 and by Act 4 of 2022. ] Sexual offences with female idiots or imbeciles
Combating of Rape Act 8 of 2000
under section 2 or referred to in section 3A shall, notwithstanding anything to the contrary contained in any other law, not run concurrently with any other sentences of imprisonment imposed on the convicted person in respect of such offence or with any part of such sentences, unless the presiding officer finds that the cumulative effect of the sentences running consecutively would be so harsh and unreasonable as to constitute cruel, inhuman or degrading punishment. [Section 3B is inserted by Act 4 of 2022.] No rule as to incapacity of boy under fourteen years to have sexual intercourse shall operate
Combating of Rape Act 8 of 2000
] (2) If a court is satisfied that substantial and compelling circumstances exist which justify the imposition of a lesser sentence than the applicable sentence prescribed in subsection (1), it shall enter those circumstances on the record of the proceedings and may thereupon impose such lesser sentence. (2A) For the purposes of subsection (2) - (a) the complainant’s character, sexual reputation or previous sexual conduct or experience; (b) an apparent lack of physical injury to the complainant apart from the rape itself; (c) any cultural or religious beliefs about rape or consent to sexual intercourse or other sexual acts held by the convicted person; or (d) any relationship between the con...
Combating of Immoral Practices Act 21 of 1980
(1) Any person who - (a) takes any female to any house or place or detains her there against her will so that any male, whether a particular male or not, may have unlawful carnal intercourse with her; or (b) takes any female to a brothel or detains her there against her will, shall be guilty of an offence and liable on conviction to imprisonment for a period not exceeding seven years. (2) When in any prosecution in terms of subsection (1) it is proved that a female was in any house or at any place so that any male, whether a particular male or not, could have unlawful carnal intercourse with her, or was in any brothel, it shall be deemed that she was taken or detained there against her will...
Combating of Immoral Practices Act 21 of 1980
“unlawful carnal intercourse” means carnal intercourse between persons who are not married or who are not partners in a customary union in terms of the traditional laws and customs applied by a particular population group referred to in section 3 of the Representative Authorities Proclamation, 1980 (Procl amation AG. 8 of 1980). [The Representative Authorities Proclamation, AG 8 of 1980, was repealed by Article 144 of the Namibian Constitution.]