It matters not if you are growing your vegetables from seed or small veg plants, whether to plant out in the veg plot depends on the weather, and your plot.
The hardiest of the Bean family are Broad beans and they can be planted out first. In March, you can sow outside if the soil is warm enough but you will need to protect new growth from a heavy frost or snow by a fleece or cloche. An alternative if you have a conservatory, greenhouse, or cold frame is to sow seed and under glass so you have sturdy plants to put into the veg plot later in the Spring.
The same applies if you are buying small vegetable plants, they will need protection from the worst of the weather. Small plug plants are good value, but they are grown in parts of the garden centre which are protected from the weather and after purchasing you will need to so the same. All Beans like a long root run are best germinated into root trainers or loo rolls holders. Sow seed nearer the top of the holder so the roots can reach down. The same is true if you are sowing sweet peas. Beans need no attention whilst growing on other than to ensure light levels are good so that plants do not get spindly. Water carefully if in loo rolls holders, only in the centre, try not to water the cardboard holder as it will disintegrate.
Many vegetables can be sown from seed in March depending on the position of your plot and either grown on under glass or covered by cloches. The same is true of small vegetable plants which can be planted purchased and grown on in a greenhouse or under glass.
If your plot is exposed/subject to frosts and cold winds even with cloches and protection available, it is best to delay outside sowing of French beans, cucumbers and squashes until later April/May. You can sow these under glass and start off young plants but for outside sowing or planting out it is better to wait for later in the season.