Hiya,
sounds like the same ERW tubing we use, but we use the 4mm size, (32mm o/d). I've never built a full chop frame, only trikes, but the headstock area of a trike frame will suffer much stronger stresses than a solo bike frame cos you can't lean a trike into corners, & we've not had any problems yet.
Personally I'd say it'd be fine, provided you design it properly with plenty of triangulated bracing, but then it's you that'll be riding it, not me. If you're still unsure, go with better grade steel, in which case you can use a more lightweight tube, but it'll cost you 3 times as much.
3mm wall ERW will give you a good solid frame. If you've put it together properly it'll take a major tarmac/car/brick wall interface to break it -in which case the state of your frame will be the least of your worries.
