customer asks about scope 1 2 and 3 emissions, how do I fill that in correctly

Scope 1, 2 and 3 are three separate questions

The customer is in fact asking three different things, even though they fall under one heading "CO2 emissions". Scope 1 is the direct emissions from the company itself: own combustion, own vehicle fleet, own production process. Scope 2 is the indirect emissions from purchased energy, usually electricity and sometimes district heating. Scope 3 is the rest of the value chain: everything suppliers emit to produce your raw materials, and everything that happens after delivery in transport and use by the customer. For scope 1 and 2, in most cases a calculation based on total energy consumption per yearconsumption is sufficient, split by fuel and electricity, with a standard emission factor per unit.

Scope 3 is the part where SMEs often get stuck, because it requires data from others. If you don't have it, then "not yet calculated" is a more valid answer than an estimated figure without substantiation. Some customers accept a qualitative explanation for scope 3 instead of a figure, especially if the company is small. Check if in doubt what exactly falls under scope 3 before you fill in anything, because part of scope 3 overlaps with questions about waste and reduction targets, and these are often separated in the same questionnaire. See also what a customer exactly means by scope 2, because "purchased energy" is sometimes confused with total energy consumption, which leads to double counting.

Where this classification comes from

The scope classification comes from the GHG Protocol, the international standard that most questionnaires use as a basis, even if they don't mention it by name. The EFRAG VSME standard, the voluntary basic module for non-listed SMEs, aligns with this and indicates what level of detail is sufficient for smaller companies: scope 1 and 2 mandatory, scope 3 often as a supplementary or voluntary indicator.