These rules have not changed substantially, although it's my experience that a hard fight is much more fun with a lot of equal-level enemies rather than a few Level +4 enemies.
Your quote refers to the distinction between monster level and encounter level. As you probably know, an encounter has a level that is used to determine its XP budget. You then spend that budget on monsters to populate the encounter.
The guideline encourages you to only use monsters that are within 3 levels of the encounter's level. Combined with the Party Level -2 to +4 restriction, that gives you a range of effective monster levels for each encounter level.
The Full Process
Excerpted rom the Rules Compendium, p. 285:
- Choose an encounter level.
- An easy encounter is one or two levels lower than the group’s level.
- A standard encounter is of the group’s level or one level higher.
- A hard encounter is two to four levels higher than the group’s level.
- Determine the XP budget. [...]
- Spend the XP budget. Choose threats that are within three levels of the encounter level.
While it doesn't say so there, I believe there's somewhere in the rules where it clarifies that monsters should also fall into the party level -2 to +4 range.
You mention that English isn't your first language. In English, "X is within Y units of Z" means that the absolute value of (X - Y) is less than or equal to Z:
|(X - Y)| <= Z To put it another way, the distance from X to Y must be Z or less. So "threats within three levels of the encounter level" means that the monster can be three levels lower than the encounter, three levels higher, or anywhere in between.
A Concrete Example
- Imagine that you have a party of Party Level 10. Valid Encounter Levels for that party are EL 8-14 (10 - 2 to 10 + 4). Additionally, valid monsters for that party fall into Monster Levels 8-14.
- Set your encounter budget based on that level. For four party members at EL 8, you want a budget of 1400. For four party members at EL 13, you want a budget of 3200.
- Select monsters to fit that budget. Here is where the guideline you mention comes in. Your monsters should be within 3 levels of your Encounter Level.
- So your EL 8 encounter can have monsters as high as level 11. It could have monsters as low as 5 *except that those fall outside the party's range established in Step 1. This means that an EL8 encounter for a L10 party can have monsters level 8-11.
- Likewise, your EL13 encounter can have monsters as low as L10 (13 - 3). It could have monsters as high as 16, except that this falls outside the party's range established in Step 1. So an EL13 encounter for a L10 party can have monsters level 10-14.
This guideline is telling you to limit the range of monsters so that you're not using a huge number of lower-level monsters with a higher-level XP budget, or spending the entire budget of a low-level encounter on one boring normal high-level monster.
I believe you'll find that the Encounter Templates in the DMG mostly follow these rules. A guideline that the designers probably considered implicit was made explicit in the Compendium.