Likely what you're missing here is that the ECs have to be under the same PO. A Performance Objective (PO) is subdivided by multiple Enabling Objectives (EO) and each EO has an EC and each PO has a PC. PCs are a big deal, like failing an exam in school. Your ECs show how well you are grasping the content, so if you fail multiple ECs in the same PO, its an indicator that you likely would not be successful on the PC. Failing an EC in month 1, another in month 4 and another in month 10 (provided they aren't the same PO) shouldn't be a big deal, unless your school is very draconian in how it deals with EC failures.
Standards is who decides what constitutes PRBs, and they don't do that in a vacuum. The Army training is all run by CTC and courses all generally follow the same format.
I've personally never seen someone fail from ECs, but usually they've failed multiple ECs over a few POs and failed a PC twice which launched the PRB. You'll find the standards briefs are to scare you into paying attention, but the goal of your course staff is to make sure you learn the material. If you don't understand something and feel uneasy about an EC/PC, you need to make your staff aware and get help from them or another student who has the material down pat.
Standards in their inbrief should have covered exactly how your school applies Initial and Formal warnings, and what would get you where on the ladder. Typically I've seen Initials are EC failures, Formals are PC failures and EC retest failures, and PRBs are for PC restest failures and serious academic misconduct (cheating).
Course TPs are available on the CTC Gagetown DWAN site if you're in the Army. It has every career course and every OSQ that you could do. Also has the qualification standards which is the document that the TP is derived from.