Plotting I/Io vs phi

Evaaa

1. The problem statement, all variables and given/known data

A three slit system is illuminated by light with a wavelength of 650 nm. The slits are evenly spaced with a slit space of 1.28 mm and a slit width of 0.16 mm

A) Using Excel, plot the modulated interference pattern for +or - 0.5° or so. Plot I/Io.
b) Are there any missing orders?
c) Draw a phasor diagram for the second minima from the center of the interference pattern.


2. Relevant equations
im not really sure which equations to use, but i assume i should use:
I= Io[(sin²(N*phi/2)/(sin²(phi/2)]
phi=[ (2*pi)/ lamda ] * d*sin(theta)
?

3. The attempt at a solution
I figured N would equal 3 because there is three slits.
and phi would equal 256.4 after plugging in all the values (+0.5 degrees).
but i have no clue how to find Io or how to plot the graph.

i know that i have to plot I/Io vs phi, and that at phi equal to 0, i have a maximum bright fringe that should be 9Io, and then at phi equal to pi i have another bright fringe (but its intensity is Io) an then at phi equal to 2pi i have another maxmimum bright fringe.

help?