§ 3. "That the extent and incidence of purchase tax be varied so as to give effect, as from such date as may be provided by any Act of the present Session relating to finance, to provisions for charging the tax on certain importations of chargeable goods and varying the transactions that are to be treated as chargeable purchases for the purposes of the tax, increasing the scope of registration, excepting persons from the effect of registration in certain cases, and requiring in certain circumstances security for payment of the tax."