I need to think a fair bit before translating this. When делать takes the instrumental, it needs an object. See that the verb делать is in 3rd person plural, referring to the word вещи. In English it would be:
"Three things make a person happy: love, interesting work and the possibility to travel".
The first phrase would translate directly as “A person make happy three things”. The phrasing is such because the most important part of the sentence, what is being emphasised comes very last in Russian.
Note the verb. Делать счастливым means to make happy, whereas Сделать счастливым (perfective form of делать) means to beatify. I imagine the reason for why you cannot say use the perfective form, is because the imperfective form indicates that these things continue to make us happy.
Меня делает счастливым русский язык.