What is the IUPAC of an alkane, C5H12 that contains three methyl groups?

2-methylbutane

2,2-dimethylpropane

with 3 methyl groups:
2-methylbutane

with 4 methyl groups:
2,2-dimethylpropane

This is an interesting exercise in naming organic structures. Draw out a 5 C- chain alkane which is pentane. Now you will see that it is only possible to add the 3 methyl groups to carbon 2,3 and/or 4. (we will discuss why later). This woud be 2,3,4 trimethyl pentane.
There are isomers to this structure which maintain the pentane spine:
1) Add 2 methyl groups onto carbon 2 and the third onto carbon3. This is 2,2,3 trimethyl pentane.
You should be able to name the compound if the 3 methyl groups are added 2 to carbon 2 and 1 to carbon 4. You can add the methyl groups in any combination to carbons 2,3 and 4.

Thre are some futher isomers of this structure, but the compound ceases to have a  pentane spine. As examples:

1) You can add the methyl groups to carbons 1.2 and 3. The longest C-chain is now 6-C so the compound chain is hexane, and the compound would be called 3,4 dimethyl hexane.
2) You can add the methyl groups to carbons 1.2 and 4 in the pentane chain, and then the compound would be names 2,4 dimethyl hexane. Notice that you now start numbering fom the other end of the C-chain. It is not 3,5 dimethyl hexane.

3) You can add the methyl groups to carbons 1,2 and 5 of the original pentane chain. The alkane C-chain then has 7 carbons and is heptane, with 1 methyl group on carbon 3. The name would be
 3-methyl heptane.

There are more isomers to discover and name. Work methodically and draw out the chains. Do not forget that you can add 2 methyl groups to any C in the spine.

What happens if you add 4 methyl groups?
There are a number of possible isomers which maintain the pentane spine. If you add 2xmethyl to C2 and 2xmethyl to C3  You get 2,2,3,3-tetramethyl pentane. You have enough experience now to draw and name other isomers. Remember, if you add 1or2 methyl groups to C1and/or C5 of the orignal pentane spine you lose the pentane and have to call the alkane spine hexane or heptane.

This is a very brief description of what is possible. My advice is always to draw out the organic structures and count the longest C-chain.

Edit: 30seconds after submitting: I read you question as: what are the names if you add 3 or 4 methyl groups onto pentane. The other answerers have it seems done a better job of interpreting you question than I have. Sorry for confusion, if there is one, but perhaps a good exercise in organic nomenclature!!!