I am unclear on whether you are teaching students to take photographs or are introducing them to the pleasures of viewing photography. But to combine art and music in lesson is a wonderful idea, either way.
One way to approach this is to gather up a collection of photographs and music. Put the students in groups, and provide them with several photographs. Put on some music and ask each group to choose a photograph or two that the group feels "matches" the music that is playing. They will have to think and talk about the music and the photographs, the beginning, really, of analysis of the elements of both. This is an ideal group project because students will be bringing different experiences to the discussion and will learn a great deal from one another. Once the groups have made their choices, these can be shared with the class, along with the group's reasoning about the choice. This discussion can be used as the foundation to introduce concepts and principles appropriate for their age levels. What they will take away from this experience is some insight into what music and photography make them think and feel, the beginnings of an appreciation for both art forms.
What kinds of music and photography might you gather up? I would think you would want a tremendous amount of variability. For music, try some classical, some folk, some country, some rock, and even at these ages, some children's music, for example, Raffi. For photography, photos of families, of nature, of industry, of cities and rural areas would be a good start. You will want to give some thought to how the students might pair these, perhaps a rural scene with country music or a song about love with a couple kissing. Some of the associations the students make might surprise you, though.
I do not know how much time you would have to devote to a "unit" like this, but the history of photography is quite interesting, too. How many people realize that there are photographs of the Civil War? The development of the camera today has a long and fascinating path, and there are great materials on-line, for example, those of the Eastman House in Rochester, New York, which has resources available on-line.
A follow up assignment, asking each student to choose a song and a photo, along with a paragraph about how they go together will allow you to have a tangible "product" to asses, and something for the students to "publish," perhaps on a bulletin board in the classroom or in the school's hallways.
If one of the goals is to introduce students to taking their own photographs, all of this would be a good start, too. The mechanics of photography, though, would have to follow, as well as a serious emphasis on the care of one's equipment. No matter how big your budget is, cameras today are not inexpensive, and picture-taking, particularly for third graders, needs to be carefully supervised.