The 3 shopping campaigns are created to segregate the traffic into 3 different buckets of high ROI, low ROI and unknown keywords. This will not increase the budget or cost as the campaigns are non competitive in nature. They will only spend the same amount of money as a single campaign.
the budget created for shopping campaigns is a shared budget. So a shared budget of $300 for 3 campaigns means that a maximum of $300 will be spent and not $900.