Please don't make a formal complaint about a KS3 homework essay that has been unmarked for three weeks. If marking is usually timely and this delay is a one off, it will be because of public exams.
At this time of the year, most teachers are on their knees. Your child's teacher will have been sending their Year 11, 12 and 13 classes off to start public exams this week. For the last two or three weeks, they will have been bombarded with students handing them practice mock exam papers, essays, PEE paragraphs, essay plans etc etc. They will also have had GCSE speaking and listening folders called for moderation, alongside all of the paper work for those.
If it was any other time of the year, I'd suggest your child politely asks your teacher when they might expect to get their essay back, and write a note to the teacher if it doesn't materialise in a day or two.
At this time of year, I'd assume that the teacher is now trying to get on top of all of the KS3 marking that slid due to prioritising the exam classes. I'd give it until Monday, then let your child ask, then write a note if it's not returned by the next lesson.
I know that 3-4 week waits are unacceptable for parents, but schools are chaotic around this time. State schools have big classes and the mark load is just horrific. The private schools I've worked in have had a lot of parents who confuse teachers with private tutors and who expect teachers to spend lunch times, after school and evenings marking practise papers just for their child. They think it's all part of the service. It is really, really stressful.
As others said, when your child begins GCSE work, they will be the one having their work prioritised, and rightly so.