Do i need to purchase a MiniProg3 for an I2C bootloader project?
As usual: That depends.
You need a device with an I2C-interface and the ability to store the project to be downloaded to your application-board.
Since your PC does not have got an I2C interface a miniprog3 comes in handy as it contains an I2C-to-USB bridge.
Of course you could use a second PSoC-board with a SD-card interface or something similar.
You may consider getting a Kit-001 which is more expensive than a Kit -030, but it allows you to develope for PSoC1, 3 and 5 and it additionally contains a miniprog3.
There is always the I2C bridge kit -
What is the max number of bootloadable applications that can reside in flash in a multi application bootloader project?
Multiple (Dual) Application bootloader supports a max of only 2 applications in flash.
There is nothing to prevent you from modifying a basic boot kernal
to accomidate > 2 applications.