He is investing money in a company that will give him 3% of his capital for a period of ninety days

Praise
be to Allah

Firstly:

it is not permissible, when forming a partnership, to agree
that the owner of the capital will get a certain percentage of his capital,
or a set amount. Rather his share should be a percentage of the profit, such
as saying: You will get 3% of the profit.

But if it is said to him: You will get 3% of your capital, or
you will get one hundred and fifty dollars, this renders the partnership
invalid.

If, combined with that, his capital is guaranteed (and he
will not lose any of it), as you mentioned, then this transaction is a riba-based
loan.

Ibn Qudaamah (may Allah have mercy on him) said:

If the share of one of the partners is set as a specific
amount of money, or a sum of money is set along with his share – such as if
he stipulates that he will have a percentage plus ten dirhams, the
partnership becomes invalid. Ibn al-Mundhir said: All of the scholars from
whom I learned are agreed that the mudaarabah partnership is invalidated if
one or both of them stipulate that they will have a certain amount of money.
Among those from whom we learned that are Maalik, al-Awzaa‘i, al-Shaafa‘i,
Abu Thawr and Ashaab al-ra’y. End quote.

Shaykh al-Islam Ibn Taymiyah said: 

If it stipulates in the mudaarabah contract that the owner of
the money will get a certain amount of money, this is not permissible,
according to scholarly consensus. That is because transactions should be
based on fairness, and these transactions are a kind of partnership, and it
can only be a partnership if each partner gets a percentage, such as
one-third or one-half. If one of them is given a specific amount, that is
not fair, rather it is unjust. End quote from
Majmoo’ al-Fataawa, 38/83 

The Standing Committee was asked about two men, one of whom
gave the other a sum of money with which to do business for him, and they
agreed that he would give him 3% of the capital as a profit each month. 

They replied: 

Your giving the money to the trader to do business with it
and his giving you a set amount, three percent of the money, is not
permissible, because it is a kind of guaranteed profit. End quote from
Fataawa al-Lajnah al-Daa’imah,
14/318 

They were also asked about
a company that takes money from people to do business with it, and gives
them a share of 30 percent of the capital every year, and claims that it
makes a profit of 100 percent. 

They replied: 

If the company mentioned
gives the partners a set, guaranteed amount of profit, this transaction is
not permissible, because it is riba. The transaction which is permissible is
that in which each partner is given a share such as one-quarter or
one-tenth, which increases or decreases according to what happens. End quote
from Fataawa al-Lajnah al-Daa’imah, 14/321 

Conclusion: it
is not permissible to invest money in the manner asked about.

Secondly:

In a valid type
of partnership, it is essential to know the nature of the field in which the
money will be invested, and that it is permissible. It is not permissible
for the money to be invested when it is not known in which field it will be
invested; perhaps it will be invested in riba-based banks or gambling
operations, or other unlawful things.

It is not
permissible for anyone to let his love of wealth make him try to accumulate
wealth in any way, even if it is prohibited. The Prophet (blessings and
peace of Allah be upon him) said: 

“The Holy
Spirit (Jibreel) has inspired to me that no soul will die until it has
completed its appointed term and received its provision in full, so fear
Allah and do not be desperate in seeking provision, and no one of you should
be temped to seek provision by means of committing sin if it is slow in
coming to him, for that which is with Allah can only be attained by obeying
Him.” Narrated by Abu Na‘eem in al-Hilyah; classed as saheeh by
al-Albaani in Saheeh al-Jaami‘, no. 2085. 

And Allah knows
best.