You are required to attach the most recent year's tax return and provide the "total income" number from tax returns from the last 3 years, if you were required to file and filed. The only reason you can not have been required to file is if your income during that year is less than the threshold for filing (which is in the beginning of each year's Form 1040 instructions). If you did not file for one or more of those years because your income that year was below the threshold and so you were not required to file, according to the I-864 instructions, you attach a statement to that effect, and you're fine.
The sponsored immigrant's income can be counted in the household income if the immigrant is authorized to work in the US.