Trisomy is having 3 copies of one particular chromosome, in a normally diploid organism.
Triploidy is having 3 sets of all the different chromosomes
Polyploidy is having more than 3 sets.
Like, we humans characteristically pass on 23 different types of chromosomes when we reproduce, and they're numbered, #1, #2, #3 etc. We are diploid (2 sets), so we have 2 of each numbered chromosome.
Trisomy would be having 3 of one of the numbered chromosomes. Like if you have 2 copies of each 1 thru 23, and then one extra copy of number 18, you have trisomy 18.
Triploidy and polyploidy is what some plant cells normally have - 3 or more full sets of chromosomes.
I found it helps to think of chromosomes as a set of all different tools that are characteristic for a species - a hammer, a wrench, a screwdriver etc. You get one full set of tools from each parent, and so you have 2 hammers, 2 wrenches and so on ... but the sets are different colors. So you have one blue hammer and one red, one blue wrench and one red, etc. If you have trisomy, you have 2 sets of tools and one extra screwdriver. If you are triploid, you have 3 different colored complete sets of tools.
Hope that helps instead of just confusing you ...
TRISOMY: It is numerical abnormality of chromosome. When an extra copy of single chromosome present in cell, it is known as Trisomy, It generally occurs due to abnormal segragation of chromosome in Meiosis.
TRIPLOIDY: Numerical aberration involves the one additional extra chromosomal sets, which is known as Triploidy.Involves 69 chromosomes.
POLYPLOIDY: An increase in the number of haploid sets (23) of chromosomes in a cell. Poly ploidy are of two types:
Triploidy and tetraTriploidy refers to three whole sets of chromosomes in a single cell (in humans, a total of 69 chromosomes per cell); tetraploidy refers to four whole sets of chromosomes in a single cell (in humans, a total of 92 chromosomes per cell).
It is due to dispermy or unreduced gametes.