Hi Saladsandwich, it's so daunting at the beginning, but you do get used to it.
Firstly, Global Development Delay (GDD) is the term used when children are young and it isn't clear what is causing their difficulties, but it affects more than two areas of development. So, if a child has difficulty picking up small objects or gripping things, etc. they would have a 'fine motor delay'. However, if they had those difficulties, weren't walking as expected and language was behind expectations too, then they would be considered to have a 'global delay'.
Some children with GDD will catch up as they grow older. Others won't. Some children will narrow the gap that developed between them and their peers; for others the gap will widen in time. Some children will go on to have a firm diagnosis (ie. Cerebral Palsy, Autistic Spectrum Condition, Fragile X syndrome, etc.). Other children will never get a firm diagnosis and will either be labelled 'GDD' or 'Mild/Moderate/Severe/Profound Learning Difficulties.'
About the language - the Griffiths Development Scale separates out the receptive language skills (ie. processing and understanding what you hear) and the expressive language skills (ie. saying and understanding what you want to communicate). Some children have a 'matched delay' ie. they are behind equally in both areas. Other children will have a 'mismatched delay' ie. they have difficulty expressing themselves with limited sentence length or vocabularly, but they understand what others say well. Other children are very 'verbal' but don't actually understand much of what they say.
The self-care thing is probably age related. The range of expectation of 3 year olds is very broad. Even without difficulties, some children of this age will still be in nappies and others will be dry day and night, etc. So although it may be behind what you and the nursery consider appropriate, in the grand scheme of things it may not be enough to be classified as 'delayed'.
Fragile X is the most common inherited intellectual disability in boys, and to a lesser extent found in girls. It is also very specifically identifiable in a test, so it is the first thing that is tested for. Karotype means that they are looking to see that his genetic 'code' is 46XY. That is the 'normal' code for boys. Girls are 'normally' 46XX. If the karotype came back 47,XY, +21, for example, it would tell the Dr that your child is a boy with Down's Syndrome. Karotyping is a very crude test. It literally tells them how many chromosomes are present, and which if any are missing or extra. It doesn't tell the Drs whether the chromosomes themselves are as they should be.