Starting in March it is still a bit cool out, and can and does drop well below freezing at elevation. When i did 3.5 months on the AT I started in December so i had full on winter gear.
For a March start If it was me I would bring:
Head-Smartwool balaclava and smartwool beanie
Torso-Capaline 3 l/s, columbia l/s hiking shirt, pantagonia nano puff, stoic vaporshell. (if you get cold easily you might want an additional layer like a r1 pullover etc)
Legs-capaline 2 long johns, summer weight bdu pants, ecwcs goretex pants
Feet- Smartwool trekking socks, OR crocodile gaitors, and my zamberlan trekker boots
Hands- smartwool glove liners, ems fleece fingerless/ mittens, and or endeavor mitts
Towards the end of april- mid may i would send a good portion of the cold weather stuff home as it is usually plenty warm by then. (balaclava, cap 3 l/s, bdu pants, croc gaitors, ecwcs pants, fleece mittens) and replace with some convertible pants, and a smartwool tshirt.
I like the bdu pants because in the colder months i like having a fire, and since they are 50/50 cotton poly blend they wont burst into flames and leave you with giant holes from embers like the other synthetic options and they still dry fairly quickly. The ecwcs pants i just have because they are as durable as they come , though there are much lighter options out there for rain pants(i only use rain pants in late fall-early spring).