Check out zybez.net they can help you with each one of those and give you the best strategies. I cannot be much of a help, but I do know that all 3 of those skills cost a lot of money to get up.
Actually I think zybez is one of the best runescape fansites out there. They update their guides and others very frequently and more than any other fansite I've been to out there. 20M would probably get you all of the skills you want, but you may have to use a cheaper method.
Herblore, do jungle potion, make some str potions until level 30, then clean herbs (you can profit if you do this right)
Smithing, use gold ores on furnace with goldsmithing gauntlets (loss of about 2.5 m)
Construction, get level 33 then make limestone fireplace. Or make oak larders also at level 33 (faster but more expensive) (Loss varies from about 3m to 7m or more)
Smithing can be done slower by smithing mithril bars, but profiting, also done slower but profiting more , cannon balls, or wasting much money by smithing highest available item