Dear M.,
There is about 2.5 years between my oldest son and daughter. Both kids expressed an "let me do it" at various times, but my daughter was so vocal and consistently expressed it from about 2.5 to even now at 4.5. And yes, she would get frustrated, but she seemed less frustrated, when I was patient and let her make the mess with no criticism or frustration on my own part.
This is such an important time for this age.
Have you taken any early childhood family education classes (ECFE)?. Look on the website for your city's public schools and they should reference the ECFE programs in your area. This is such a terrific parenting resource. The classes are so invaluable to helping parents understand their own styles, strengths, weakness, and responsibilities as the most important role model for their kids. Plus it provides an invaluable support mechanism for parents to share common challenges, practices, ideas, and the kids meet others for playtime.
Your youngest son is nearing the age for the early screen (3.5). This is a requirement for the schools (it is free),and generally conducted by the ECFE facility. They could definitely help you to determine if your son may need some further evaluation. That is the whole intent with the early screening, it helps parents and the school community to be able to fund programs that may be needed by families and to provide additional programs at the schools too.