I have a continuous DV and 2 categorical + 1 continuous IVs. In spss I did a univariate analysis and found a main effect of one of the categorical ivs and a significant interaction between the two categorical IVs and the DV. I am interested in seeing if there is a three way interaction between the two categorical and one continuous IV, so in spss I did:

glm DV by IV1 IV2 with IV3(the continuous)/
design= IV1 IV2 IV3 IV1*IV3 IV2*IV3 IV1*IV2*IV3 IV1*IV2.

IV1*IV2*IV3 interaction was significant as well as the same IV1*IV2 as before, but now IV1 wasn't significant on its own? what is the reason for this? and did I do this three way interaction correctly?