Keeping funds at hand to buy when the price of bitcoin dips is not lump sum but buying at the dip. Don't get it twisted, lump sum is when you have your bonus and use the money to buy bitcoin right aware regardless of the price of bitcoin. Like what you said there's nothing bad in splitting the funds in two or three parts, but if you don't buy right away with one part, you haven't lump sum.
Actually, some folks misunderstood buying at a DIP and lump summing. Buying at a DIP means keeping a discretionary income while waiting for the price of bitcoin to fall before buying but lump summing is simply what we call an ''all in''. An amount you have kept to invest at once probably without investing again but only leave your investment to grow in the long term. @Frankolala, you sound a bit contradictory because for someone who wants to lump sum, there is no need of splitting funds again except for someone who want to start investing while keeping some amount to buy when a DIP occurs of which it can't be regarded as lump summing. Though an investor can still decide to split their discretionary income into three parts for DCA, lump summing and buying at a DIP.